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diff --git a/3054-h/3054-h.htm b/3054-h/3054-h.htm new file mode 100644 index 0000000..9f4f299 --- /dev/null +++ b/3054-h/3054-h.htm @@ -0,0 +1,7074 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" +"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> +<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> +<head> +<meta http-equiv="Content-Type" content="text/html;charset=utf-8" /> +<meta http-equiv="Content-Style-Type" content="text/css" /> +<title>The Project Gutenberg eBook of Geological Observations on the Volcanic Islands, by Charles Darwin</title> + +<style type="text/css"> + +body { margin-left: 20%; + margin-right: 20%; + text-align: justify; } + +h1, h2, h3, h4, h5 {text-align: center; font-style: normal; font-weight: +normal; line-height: 1.5; margin-top: .5em; margin-bottom: .5em;} + +h1 {font-size: 300%; + margin-top: 0.6em; + margin-bottom: 0.6em; + letter-spacing: 0.12em; + word-spacing: 0.2em; + text-indent: 0em;} +h2 {font-size: 150%; margin-top: 2em; margin-bottom: 1em;} +h3 {font-size: 130%; margin-top: 1em;} +h4 {font-size: 120%;} +h5 {font-size: 110%;} + +.no-break {page-break-before: avoid;} /* for epubs */ + +div.chapter {page-break-before: always; margin-top: 4em;} + +hr {width: 80%; margin-top: 2em; margin-bottom: 2em;} + +p {text-indent: 1em; + margin-top: 0.25em; + margin-bottom: 0.25em; } + +p.footnote {font-size: 90%; + text-indent: 0%; + margin-left: 10%; + margin-right: 10%; + margin-top: 1em; + margin-bottom: 1em; } + +a:link {color:blue; text-decoration:none} +a:visited {color:blue; text-decoration:none} +a:hover {color:red} + +</style> + +</head> + +<body> + +<div style='text-align:center; font-size:1.2em; font-weight:bold;'>The Project Gutenberg eBook of Geological Observations on the Volcanic Islands, by Charles Darwin</div> +<div style='display:block; margin:1em 0'> +This eBook is for the use of anyone anywhere in the United States and +most other parts of the world at no cost and with almost no restrictions +whatsoever. You may copy it, give it away or re-use it under the terms +of the Project Gutenberg License included with this eBook or online +at <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you +are not located in the United States, you will have to check the laws of the +country where you are located before using this eBook. +</div> +<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: Geological Observations on the Volcanic Islands</div> +<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Author: Charles Darwin</div> +<div style='display:block;margin:1em 0'>Release Date: February, 2002 [eBook #3054]<br /> +[Most recently updated: March 29, 2021]</div> +<div style='display:block;margin:1em 0'>Language: English</div> +<div style='display:block;margin:1em 0'>Character set encoding: UTF-8</div> +<div style='display:block; margin-left:2em; text-indent:-2em'>Produced by: Sue Asscher </div> +<div style='margin-top:2em;margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK VOLCANIC ISLANDS ***</div> + +<h1>Geological Observations on the Volcanic Islands</h1> + +<h2 class="no-break">by Charles Darwin</h2> + +<hr /> + +<h2>Contents</h2> + +<table summary="" style=""> + +<tr> +<td> <a href="#pref01">EDITORIAL NOTE</a></td> +</tr> + +<tr> +<td> <a href="#toc01">DETAILED TABLE OF CONTENTS</a></td> +</tr> + +<tr> +<td> <a href="#pref02">CRITICAL INTRODUCTION</a></td> +</tr> + +<tr> +<td> <a href="#chap01">CHAPTER I.—ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO</a></td> +</tr> + +<tr> +<td> <a href="#chap02">CHAPTER II.—FERNANDO NORONHA; TERCEIRA; TAHITI, ETC</a></td> +</tr> + +<tr> +<td> <a href="#chap03">CHAPTER III.—ASCENSION</a></td> +</tr> + +<tr> +<td> <a href="#chap04">CHAPTER IV.—ST. HELENA</a></td> +</tr> + +<tr> +<td> <a href="#chap05">CHAPTER V.—GALAPAGOS ARCHIPELAGO</a></td> +</tr> + +<tr> +<td> <a href="#chap06">CHAPTER VI.—TRACHYTE AND BASALT.—DISTRIBUTION OF VOLCANIC ISLES</a></td> +</tr> + +<tr> +<td> <a href="#chap07">CHAPTER VII.—AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE</a></td> +</tr> + +<tr> +<td> <a href="#chap08">INDEX TO VOLCANIC ISLANDS</a></td> +</tr> + +</table> + +<div class="chapter"> + +<h2><a name="pref01"></a>EDITORIAL NOTE</h2> + +<p> +Although in some respects more technical in their subjects and style than +Darwin’s “Journal,” the books here reprinted will never lose +their value and interest for the originality of the observations they contain. +Many parts of them are admirably adapted for giving an insight into problems +regarding the structure and changes of the earth’s surface, and in fact +they form a charming introduction to physical geology and physiography in their +application to special domains. The books themselves cannot be obtained for +many times the price of the present volume, and both the general reader, who +desires to know more of Darwin’s work, and the student of geology, who +naturally wishes to know how a master mind reasoned on most important +geological subjects, will be glad of the opportunity of possessing them in a +convenient and cheap form. +</p> + +<p> +The three introductions, which my friend Professor Judd has kindly furnished, +give critical and historical information which makes this edition of special +value. +</p> + +<h3>G.T.B.</h3> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2>VOLCANIC ISLANDS.</h2> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="toc01"></a>DETAILED TABLE OF CONTENTS</h2> + +<h3><a href="#chap01">I.—ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.</a></h3> + +<p> +Rocks of the lowest series.—A calcareous sedimentary deposit, with recent +shells, altered by the contact of superincumbent lava, its horizontality and +extent.—Subsequent volcanic eruptions, associated with calcareous matter +in an earthy and fibrous form, and often enclosed within the separate cells of +the scoriae.—Ancient and obliterated orifices of eruption of small +size.—Difficulty of tracing over a bare plain recent streams of +lava.—Inland hills of more ancient volcanic rock.—Decomposed +olivine in large masses.—Feldspathic rocks beneath the upper crystalline +basaltic strata.—Uniform structure and form of the more ancient volcanic +hills.—Form of the valleys near the coast.—Conglomerate now forming +on the sea beach. +</p> + +<h3><a href="#chap02">II.—FERNANDO NORONHA; TERCEIRA; TAHITI, ETC.</a></h3> + +<p> +FERNANDO NORONHA.—Precipitous hill of phonolite. +</p> + +<p> +TERCEIRA.—Trachytic rocks: their singular decomposition by steam of high +temperature. +</p> + +<p> +TAHITI.—Passage from wacke into trap; singular volcanic rock with the +vesicles half-filled with mesotype. +</p> + +<p> +MAURITIUS.—Proofs of its recent elevation.—Structure of its more +ancient mountains; similarity with St. Jago. +</p> + +<p> +ST. PAUL’S ROCKS.—Not of volcanic origin.—Their singular +mineralogical composition. +</p> + +<h3><a href="#chap03">III.—ASCENSION.</a></h3> + +<p> +Basaltic lavas.—Numerous craters truncated on the same +side.—Singular structure of volcanic bombs.—Aeriform +explosions.—Ejected granite fragments.—Trachytic +rocks.—Singular veins.—Jasper, its manner of +formation.—Concretions in pumiceous tuff.—Calcareous deposits and +frondescent incrustations on the coast.—Remarkable laminated beds, +alternating with, and passing into obsidian.—Origin of obsidian.— +Lamination of volcanic rocks. +</p> + +<h3><a href="#chap04">IV.—ST. HELENA.</a></h3> + +<p> +Lavas of the feldspathic, basaltic, and submarine series.—Section of +Flagstaff Hill and of the Barn.—Dikes.—Turk’s Cap and +Prosperous Bays.— Basaltic ring.—Central crateriform ridge, with an +internal ledge and a parapet.—Cones of phonolite.—Superficial beds +of calcareous sandstone.— Extinct land-shells.—Beds of +detritus.—Elevation of the land.— Denudation.—Craters of +elevation. +</p> + +<h3><a href="#chap05">V.—GALAPAGOS ARCHIPELAGO.</a></h3> + +<p> +Chatham Island.—Craters composed of a peculiar kind of tuff.—Small +basaltic craters, with hollows at their bases.—Albemarle Island; fluid +lavas, their composition.—Craters of tuff; inclination of their exterior +diverging strata, and structure of their interior converging +strata.—James Island, segment of a small basaltic crater; fluidity and +composition of its lava-streams, and of its ejected fragments.—Concluding +remarks on the craters of tuff, and on the breached condition of their southern +sides.— Mineralogical composition of the rocks of the +archipelago.—Elevation of the land.—Direction of the fissures of +eruption. +</p> + +<h3><a href="#chap06">VI.—TRACHYTE AND BASALT.—DISTRIBUTION OF VOLCANIC ISLES.</a></h3> + +<p> +The sinking of crystals in fluid lava.—Specific gravity of the +constituent parts of trachyte and of basalt, and their consequent +separation.— Obsidian.—Apparent non-separation of the elements of +plutonic rocks.— Origin of trap-dikes in the plutonic +series.—Distribution of volcanic islands; their prevalence in the great +oceans.—They are generally arranged in lines.—The central volcanoes +of Von Buch doubtful.—Volcanic islands bordering +continents.—Antiquity of volcanic islands, and their elevation in +mass.—Eruptions on parallel lines of fissure within the same geological +period. +</p> + +<h3><a href="#chap07">VII.—AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE.</a></h3> + +<p> +New South Wales.—Sandstone formation.—Embedded pseudo-fragments of +shale.—Stratification.—Current-cleavage.—Great +valleys.—Van Diemen’s Land.—Palaeozoic formation.—Newer +formation with volcanic rocks.— Travertin with leaves of extinct +plants.—Elevation of the land.—New Zealand.—King +George’s Sound.—Superficial ferruginous beds.—Superficial +calcareous deposits, with casts of branches; its origin from drifted particles +of shells and corals.—Their extent.—Cape of Good Hope.— +Junction of the granite and clay-slate.—Sandstone formation. +</p> + +<h3><a href="#chap08">INDEX.</a></h3> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2>GEOLOGICAL OBSERVATIONS ON VOLCANIC ISLANDS.</h2> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="pref02"></a>CRITICAL INTRODUCTION.</h2> + +<p> +The preparation of the series of works published under the general title +“Geology of the Voyage of the ‘Beagle’” occupied a +great part of Darwin’s time during the ten years that followed his return +to England. The second volume of the series, entitled “Geological +Observations on Volcanic Islands, with Brief Notices on the Geology of +Australia and the Cape of Good Hope,” made its appearance in 1844. The +materials for this volume were collected in part during the outward voyage, +when the “Beagle” called at St. Jago in the Cape de Verde Islands, +and St. Paul’s Rocks, and at Fernando Noronha, but mainly during the +homeward cruise; then it was that the Galapagos Islands were surveyed, the Low +Archipelago passed through, and Tahiti visited; after making calls at the Bay +of Islands, in New Zealand, and also at Sydney, Hobart Town and King +George’s Sound in Australia, the “Beagle” sailed across the +Indian Ocean to the little group of the Keeling or Cocos Islands, which Darwin +has rendered famous by his observations, and thence to Mauritius; calling at +the Cape of Good Hope on her way, the ship then proceeded successively to St. +Helena and Ascension, and revisited the Cape de Verde Islands before finally +reaching England. +</p> + +<p> +Although Darwin was thus able to gratify his curiosity by visits to a great +number of very interesting volcanic districts, the voyage opened for him with a +bitter disappointment. He had been reading Humboldt’s “Personal +Narrative” during his last year’s residence in Cambridge, and had +copied out from it long passages about Teneriffe. He was actually making +inquiries as to the best means of visiting that island, when the offer was made +to him to accompany Captain Fitzroy in the “Beagle. “ His friend +Henslow too, on parting with him, had given him the advice to procure and read +the recently published first volume of the “Principles of Geology,” +though he warned him against accepting the views advocated by its author. +During the time the “Beagle” was beating backwards and forwards +when the voyage commenced, Darwin, although hardly ever able to leave his +berth, was employing all the opportunities which the terrible sea-sickness left +him, in studying Humboldt and Lyell. We may therefore form an idea of his +feelings when, on the ship reaching Santa Cruz, and the Peak of Teneriffe +making its appearance among the clouds, they were suddenly informed that an +outbreak of cholera would prevent any landing! +</p> + +<p> +Ample compensation for this disappointment was found, however, when the ship +reached Porta Praya in St. Jago, the largest of the Cape de Verde Islands. Here +he spent three most delightful weeks, and really commenced his work as a +geologist and naturalist. Writing to his father he says, “Geologising in +a volcanic country is most delightful; besides the interest attached to itself, +it leads you into most beautiful and retired spots. Nobody but a person fond of +Natural History can imagine the pleasure of strolling under cocoa-nuts in a +thicket of bananas and coffee-plants, and an endless number of wild flowers. +And this island, that has given me so much instruction and delight, is reckoned +the most uninteresting place that we perhaps shall touch at during our voyage. +It certainly is generally very barren, but the valleys are more exquisitely +beautiful, from the very contrast. It is utterly useless to say anything about +the scenery; it would be as profitable to explain to a blind man colours, as to +a person who has not been out of Europe, the total dissimilarity of a tropical +view. Whenever I enjoy anything, I always look forward to writing it down, +either in my log-book (which increases in bulk), or in a letter; so you must +excuse raptures, and those raptures badly expressed. I find my collections are +increasing wonderfully, and from Rio I think I shall be obliged to send a cargo +home.” +</p> + +<p> +The indelible impression made on Darwin’s mind by this first visit to a +volcanic island, is borne witness to by a remarkable passage in the +“Autobiography” written by him in 1876. “The geology of St. +Jago is very striking, yet simple; a stream of lava formerly flowed over the +bed of the sea, formed of triturated recent shells and corals, which it has +baked into a hard white rock. Since then the whole island has been upheaved. +But the line of white rock revealed to me a new and important fact, namely that +there had been afterwards subsidence round the craters which had since been in +action, and had poured forth lava. It then first dawned on me that I might +perhaps write a book on the geology of the various countries visited, and this +made me thrill with delight. That was a memorable hour to me, and how +distinctly I can call to mind the low cliff of lava beneath which I rested, +with the sun glaring hot, a few strange desert plants growing near and with +living corals in the tidal pools at my feet.” +</p> + +<p> +Only five years before, when listening to poor Professor Jameson’s +lectures on the effete Wernerianism, which at that time did duty for geological +teaching, Darwin had found them “incredibly dull,” and he declared +that “the sole effect they produced on me was a determination never so +long as I lived to read a book on Geology, or in any way to study the +science.” +</p> + +<p> +What a contrast we find in the expressions which he makes use of in referring +to Geological Science, in his letters written home from the +“Beagle!” After alluding to the delight of collecting and studying +marine animals, he exclaims, “But Geology carries the day!” Writing +to Henslow he says, “I am quite charmed with Geology, but, like the wise +animal between two bundles of hay, I do not know which to like best; the old +crystalline group of rocks, or the softer and more fossiliferous beds.” +And just as the long voyage is about to come to a close he again writes, +“I find in Geology a never-failing interest; as it has been remarked, it +creates the same grand ideas respecting this world which Astronomy does for the +Universe.” In this passage Darwin doubtless refers to a remark of Sir +John Herschel’s in his admirable “Preliminary Discourse on the +Study of Natural Philosophy,”—a book which exercised a most +remarkable and beneficial influence on the mind of the young naturalist. +</p> + +<p> +If there cannot be any doubt as to the strong predilection in Darwin’s +mind for geological studies, both during and after the memorable voyage, there +is equally little difficulty in perceiving the school of geological thought +which, in spite of the warnings of Sedgwick and Henslow, had obtained complete +ascendancy over his mind. He writes in 1876: “The very first place which +I examined, namely St. Jago in the Cape de Verde Islands, showed me clearly the +wonderful superiority of Lyell’s manner of treating Geology, compared +with that of any other author, whose works I had with me, or ever afterwards +read.” And again, “The science of Geology is enormously indebted to +Lyell—more so, as I believe, than to any other man who ever lived...I am +proud to remember that the first place, namely, St. Jago, in the Cape de Verde +Archipelago, in which I geologised, convinced me of the infinite superiority of +Lyell’s views over those advocated in any other work known to me.” +</p> + +<p> +The passages I have cited will serve to show the spirit in which Darwin entered +upon his geological studies, and the perusal of the following pages will +furnish abundant proofs of the enthusiasm, acumen, and caution with which his +researches were pursued. +</p> + +<p> +Large collections of rocks and minerals were made by Darwin during his +researches, and sent home to Cambridge, to be kept under the care of his +faithful friend Henslow. After visiting his relations and friends, +Darwin’s first care on his return to England was to unpack and examine +these collections. He accordingly, at the end of 1836, took lodgings for three +months in Fitzwilliam Street, Cambridge, so as to be near Henslow; and in +studying and determining his geological specimens received much valuable aid +from the eminent crystallographer and mineralogist, Professor William Hallows +Miller. +</p> + +<p> +The actual writing of the volume upon volcanic islands was not commenced till +1843, when Darwin had settled in the spot which became his home for the rest of +his life—the famous house at Down, in Kent. Writing to his friend Mr. +Fox, on March 28th, 1843, he says, “I am very slowly progressing with a +volume, or rather pamphlet, on the volcanic islands which we visited: I manage +only a couple of hours per day, and that not very regularly. It is uphill work +writing books, which cost money in publishing, and which are not read even by +geologists.” +</p> + +<p> +The work occupied Darwin during the whole of the year 1843, and was issued in +the spring of the following year, the actual time engaged in preparing it being +recorded in his diary as “from the summer of 1842 to January 1844;” +but the author does not appear to have been by any means satisfied with the +result when the book was finished. He wrote to Lyell, “You have pleased +me much by saying that you intend looking through my ‘Volcanic +Islands;’ it cost me eighteen months!!! and I have heard of very few who +have read it. Now I shall feel, whatever little (and little it is) there is +confirmatory of old work, or new, will work its effect and not be lost.” +To Sir Joseph Hooker he wrote, “I have just finished a little volume on +the volcanic islands which we visited. I do not know how far you care for dry +simple geology, but I hope you will let me send you a copy.” +</p> + +<p> +Every geologist knows how full of interest and suggestiveness is this book of +Darwin’s on volcanic islands. Probably the scant satisfaction which its +author seemed to find in it may be traced to the effect of a contrast which he +felt between the memory of glowing delights he had experienced when, hammer in +hand, he roamed over new and interesting scenes, and the slow, laborious, and +less congenial task of re-writing and arranging his notes in book-form. +</p> + +<p> +In 1874, in writing an account of the ancient volcanoes of the Hebrides, I had +frequent occasion to quote Mr. Darwin’s observations on the Atlantic +volcanoes, in illustration of the phenomena exhibited by the relics of still +older volcanoes in our own islands. Darwin, in writing to his old friend Sir +Charles Lyell upon the subject, says, “I was not a little pleased to see +my volcanic book quoted, for I thought it was completely dead and +forgotten.” +</p> + +<p> +Two years later the original publishers of this book and of that on South +America proposed to re-issue them. Darwin at first hesitated, for he seemed to +think there could be little of abiding interest in them; he consulted me upon +the subject in one of the conversations which I used to have with him at that +time, and I strongly urged upon him the reprint of the works. I was much +gratified when he gave way upon the point, and consented to their appearing +just as originally issued. In his preface he says, “Owing to the great +progress which Geology has made in recent times, my views on some few points +may be somewhat antiquated, but I have thought it best to leave them as they +originally appeared.” +</p> + +<p> +It may be interesting to indicate, as briefly as possible, the chief geological +problem upon which the publication of Darwin’s “Volcanic +Islands” threw new and important light. The merit of the work consisted +in supplying interesting observations, which in some cases have proved of +crucial value in exploding prevalent fallacies; in calling attention to +phenomena and considerations that had been quite overlooked by geologists, but +have since exercised an important influence in moulding geological speculation; +and lastly in showing the importance which attaches to small and seemingly +insignificant causes, some of which afford a key to the explanation of very +curious geological problems. +</p> + +<p> +Visiting as he did the districts in which Von Buch and others had found what +they thought to be evidence of the truth of “Elevation-craters,” +Darwin was able to show that the facts were capable of a totally different +interpretation. The views originally put forward by the old German geologist +and traveller, and almost universally accepted by his countrymen, had met with +much support from Elie de Beaumont and Dufrenoy, the leaders of geological +thought in France. They were, however, stoutly opposed by Scrope and Lyell in +this country, and by Constant Prevost and Virlet on the other side of the +channel. Darwin, in the work before us, shows how little ground there is for +the assumption that the great ring-craters of the Atlantic islands have +originated in gigantic blisters of the earth’s surface which, opening at +the top, have given origin to the craters. Admitting the influence of the +injection of lava into the structure of the volcanic cones, in increasing their +bulk and elevation, he shows that, in the main, the volcanoes are built up by +repeated ejections causing an accumulation of materials around the vent. +</p> + +<p> +While, however, agreeing on the whole with Scrope and Lyell, as to the +explosive origin of ordinary volcanic craters, Darwin clearly saw that, in some +cases, great craters might be formed or enlarged, by the subsidence of the +floors after eruptions. The importance of this agency, to which too little +attention has been directed by geologists, has recently been shown by Professor +Dana, in his admirable work on Kilauea and the other great volcanoes of the +Hawaiian Archipelago. +</p> + +<p> +The effects of subsidence at a volcanic centre in producing a downward dip of +the strata around it, was first pointed out by Darwin, as the result of his +earliest work in the Cape de Verde Islands. Striking illustrations of the same +principle have since been pointed out by M. Robert and others in Iceland, by +Mr. Heaphy in New Zealand, and by myself in the Western Isles of Scotland. +</p> + +<p> +Darwin again and again called attention to the evidence that volcanic vents +exhibit relations to one another which can only be explained by assuming the +existence of lines of fissure in the earth’s crust, along which the lavas +have made their way to the surface. But he, at the same time, clearly saw that +there was no evidence of the occurrence of great deluges of lava along such +fissures; he showed how the most remarkable plateaux, composed of successive +lava sheets, might be built up by repeated and moderate ejections from numerous +isolated vents; and he expressly insists upon the rapidity with which the +cinder-cones around the orifices of ejection and the evidences of successive +outflows of lava would be obliterated by denudation. +</p> + +<p> +One of the most striking parts of the book is that in which he deals with the +effects of denudation in producing “basal wrecks” or worn down +stumps of volcanoes. He was enabled to examine a series of cases in which could +be traced every gradation, from perfect volcanic cones down to the solidified +plugs which had consolidated in the vents from which ejections had taken place. +Darwin’s observations on these points have been of the greatest value and +assistance to all who have essayed to study the effects of volcanic action +during earlier periods of the earth’s history. Like Lyell, he was firmly +persuaded of the continuity of geological history, and ever delighted in +finding indications, in the present order of nature, that the phenomena of the +past could be accounted for by means of causes which are still in operation. +Lyell’s last work in the field was carried on about his home in +Forfarshire, and only a few months before his death he wrote to Darwin: +“All the work which I have done has confirmed me in the belief that the +only difference between Palaeozoic and recent volcanic rocks is no more than we +must allow for, by the enormous time to which the products of the oldest +volcanoes have been subjected to chemical changes.” +</p> + +<p> +Darwin was greatly impressed, as the result of his studies of volcanic +phenomena, followed by an examination of the great granite-masses of the Andes, +with the relations between the so-called Plutonic rocks and those of +undoubtedly volcanic origin. It was indeed a fortunate circumstance, that after +studying some excellent examples of recent volcanic rocks, he proceeded to +examine in South America many fine illustrations of the older igneous +rock-masses, and especially of the most highly crystalline types of the same, +and then on his way home had opportunities of reviving the impression made upon +him by the fresh and unaltered volcanic rocks. Some of the general +considerations suggested by these observations were discussed in a paper read +by him before the Geological Society, on March 7th, 1838, under the title +“On the Connection of Certain Volcanic Phenomena, and On the Formation of +Mountain-chains, and the Effect of Continental Elevations.” The exact +bearing of these two classes of facts upon one another are more fully discussed +in his book on South American geology. +</p> + +<p> +The proofs of recent elevation around many of the volcanic islands led Darwin +to conclude that volcanic areas were, as a rule, regions in which upward +movements were taking place, and he was naturally led to contrast them with the +areas in which, as he showed, the occurrence of atolls, encircling reefs, and +barrier-reefs afford indication of subsidence. In this way he was able to map +out the oceanic areas in different zones, along which opposite kinds of +movement were taking place. His conclusions on this subject were full of +novelty and suggestiveness. +</p> + +<p> +Very clearly did Darwin recognise the importance of the fact that most of the +oceanic islands appear to be of volcanic origin, though he was careful to point +out the remarkable exceptions which somewhat invalidate the generalisation. In +his “Origin of Species” he has elaborated the idea and suggested +the theory of the permanence of ocean-basins, a suggestion which has been +adopted and pushed farther by subsequent authors, than we think its originator +would have approved. His caution and fairness of mind on this and similar +speculative questions was well-known to all who were in the habit of discussing +them with him. +</p> + +<p> +Some years before the voyage of the “Beagle,” Mr. Poulett Scrope +had pointed out the remarkable analogies that exist between certain igneous +rocks of banded structure, as seen in the Ponza Islands, and the foliated +crystalline schists. It does not appear that Darwin was acquainted with this +remarkable memoir, but quite independently he called attention to the same +phenomena when he came to study some very similar rocks which occur in the +island of Ascension. Coming fresh from the study of the great masses of +crystalline schist in the South American continent, he was struck by the +circumstance that in the undoubtedly igneous rocks of Ascension we find a +similar separation of the constituent minerals along parallel +“folia.” These observations led Darwin to the same conclusion as +that arrived at some time before by Scrope—namely that when +crystallisation takes place in rock masses under the influence of great +deforming stresses, a separation and parallel arrangement of the constituent +minerals will result. This is a process which is now fully recognised as having +been a potent factor in the production of the metamorphic rock, and has been +called by more recent writers “dynamo-metamorphism.” +</p> + +<p> +In this, and in many similar discussions, in which exact mineralogical +knowledge was required, it is remarkable how successful Darwin was in making +out the true facts with regard to the rocks he studied by the simple aid of a +penknife and pocket-lens, supplemented by a few chemical tests and the constant +use of the blowpipe. Since his day, the method of study of rocks by thin +sections under the microscope has been devised, and has become a most efficient +aid in all petrographical inquiries. During the voyage of H.M.S. +“Challenger,” many of the islands studied by Darwin have been +revisited and their rocks collected. The results of their study by one of the +greatest masters of the science of micropetrography—Professor Renard of +Brussels—have been recently published in one of the volumes of +“Reports on the ‘Challenger’ Expedition.” While much +that is new and valuable has been contributed to geological science by these +more recent investigations, and many changes have been made in nomenclature and +other points of detail, it is interesting to find that all the chief facts +described by Darwin and his friend Professor Miller have stood the test of time +and further study, and remain as a monument of the acumen and accuracy in +minute observation of these pioneers in geological research. +</p> + +<h3>JOHN W. JUDD.</h3> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap01"></a>CHAPTER I.<br /> +ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.</h2> + +<p> +Rocks of the lowest series. A calcareous sedimentary deposit, with recent +shells, altered by the contact of superincumbent lava, its horizontality and +extent. Subsequent volcanic eruptions, associated with calcareous matter in an +earthy and fibrous form, and often enclosed within the separate cells of the +scoriae. Ancient and obliterated orifices of eruption of small size. Difficulty +of tracing over a bare plain recent streams of lava. Inland hills of more +ancient volcanic rock. Decomposed olivine in large masses. Feldspathic rocks +beneath the upper crystalline basaltic strata. Uniform structure and form of +the more ancient volcanic hills. Form of the valleys near the coast. +Conglomerate now forming on the sea beach. +</p> + +<h3>(FIGURE 1: MAP 1: PART OF ST. JAGO, ONE OF THE CAPE DE VERDE ISLANDS.) +</h3> + +<p> +The island of St. Jago extends in a N.N.W. and S.S.E. direction, thirty miles +in length by about twelve in breadth. My observations, made during two visits, +were confined to the southern portion within the distance of a few leagues from +Porto Praya. The country, viewed from the sea, presents a varied outline: +smooth conical hills of a reddish colour (like Red Hill in Figure 1 (Map 1). +(The outline of the coast, the position of the villages, streamlets, and of +most of the hills in this woodcut, are copied from the chart made on board +H.M.S. “Leven.” The square-topped hills (A, B, C, etc.) are put in +merely by eye, to illustrate my description.)), and others less regular, +flat-topped, and of a blackish colour (like A, B, C,) rise from successive, +step-formed plains of lava. At a distance, a chain of mountains, many thousand +feet in height, traverses the interior of the island. There is no active +volcano in St. Jago, and only one in the group, namely at Fogo. The island +since being inhabited has not suffered from destructive earthquakes. +</p> + +<p> +The lowest rocks exposed on the coast near Porto Praya, are highly crystalline +and compact; they appear to be of ancient, submarine, volcanic origin; they are +unconformably covered by a thin, irregular, calcareous deposit, abounding with +shells of a late tertiary period; and this again is capped by a wide sheet of +basaltic lava, which has flowed in successive streams from the interior of the +island, between the square-topped hills marked A, B, C, etc. Still more recent +streams of lava have been erupted from the scattered cones, such as Red and +Signal Post Hills. The upper strata of the square-topped hills are intimately +related in mineralogical composition, and in other respects, with the lowest +series of the coast- rocks, with which they seem to be continuous. +</p> + +<h3>MINERALOGICAL DESCRIPTION OF THE ROCKS OF THE LOWEST SERIES.</h3> + +<p> +These rocks possess an extremely varying character; they consist of black, +brown, and grey, compact, basaltic bases, with numerous crystals of augite, +hornblende, olivine, mica, and sometimes glassy feldspar. A common variety is +almost entirely composed of crystals of augite with olivine. Mica, it is known, +seldom occurs where augite abounds; nor probably does the present case offer a +real exception, for the mica (at least in my best characterised specimen, in +which one nodule of this mineral is nearly half an inch in length) is as +perfectly rounded as a pebble in a conglomerate, and evidently has not been +crystallised in the base, in which it is now enclosed, but has proceeded from +the fusion of some pre-existing rock. These compact lavas alternate with tuffs, +amygdaloids, and wacke, and in some places with coarse conglomerate. Some of +the argillaceous wackes are of a dark green colour, others, pale +yellowish-green, and others nearly white; I was surprised to find that some of +the latter varieties, even where whitest, fused into a jet black enamel, whilst +some of the green varieties afforded only a pale gray bead. Numerous dikes, +consisting chiefly of highly compact augitic rocks, and of gray amygdaloidal +varieties, intersect the strata, which have in several places been dislocated +with considerable violence, and thrown into highly inclined positions. One line +of disturbance crosses the northern end of Quail Island (an islet in the Bay of +Porto Praya), and can be followed to the mainland. These disturbances took +place before the deposition of the recent sedimentary bed; and the surface, +also, had previously been denuded to a great extent, as is shown by many +truncated dikes. +</p> + +<p> +DESCRIPTION OF THE CALCAREOUS DEPOSIT OVERLYING THE FOREGOING VOLCANIC ROCKS. +</p> + +<p> +This stratum is very conspicuous from its white colour, and from the extreme +regularity with which it ranges in a horizontal line for some miles along the +coast. Its average height above the sea, measured from the upper line of +junction with the superincumbent basaltic lava, is about sixty feet; and its +thickness, although varying much from the inequalities of the underlying +formation, may be estimated at about twenty feet. It consists of quite white +calcareous matter, partly composed of organic debris, and partly of a substance +which may be aptly compared in appearance with mortar. Fragments of rock and +pebbles are scattered throughout this bed, often forming, especially in the +lower part, a conglomerate. Many of the fragments of rock are whitewashed with +a thin coating of calcareous matter. At Quail Island, the calcareous deposit is +replaced in its lowest part by a soft, brown, earthy tuff, full of Turritellae; +this is covered by a bed of pebbles, passing into sandstone, and mixed with +fragments of echini, claws of crabs, and shells; the oyster-shells still +adhering to the rock on which they grew. Numerous white balls appearing like +pisolitic concretions, from the size of a walnut to that of an apple, are +embedded in this deposit; they usually have a small pebble in their centres. +Although so like concretions, a close examination convinced me that they were +Nulliporae, retaining their proper forms, but with their surfaces slightly +abraded: these bodies (plants as they are now generally considered to be) +exhibit under a microscope of ordinary power, no traces of organisation in +their internal structure. Mr. George R. Sowerby has been so good as to examine +the shells which I collected: there are fourteen species in a sufficiently +perfect condition for their characters to be made out with some degree of +certainty, and four which can be referred only to their genera. Of the fourteen +shells, of which a list is given in the Appendix, eleven are recent species; +one, though undescribed, is perhaps identical with a species which I found +living in the harbour of Porto Praya; the two remaining species are unknown, +and have been described by Mr. Sowerby. Until the shells of this Archipelago +and of the neighbouring coasts are better known, it would be rash to assert +that even these two latter shells are extinct. The number of species which +certainly belong to existing kinds, although few in number, are sufficient to +show that the deposit belongs to a late tertiary period. From its mineralogical +character, from the number and size of the embedded fragments, and from the +abundance of Patellae, and other littoral shells, it is evident that the whole +was accumulated in a shallow sea, near an ancient coast-line. +</p> + +<p> +EFFECTS PRODUCED BY THE FLOWING OF THE SUPERINCUMBENT BASALTIC LAVA OVER THE +CALCAREOUS DEPOSIT. +</p> + +<p> +These effects are very curious. The calcareous matter is altered to the depth +of about a foot beneath the line of junction; and a most perfect gradation can +be traced, from loosely aggregated, small, particles of shells, corallines, and +Nulliporae, into a rock, in which not a trace of mechanical origin can be +discovered, even with a microscope. Where the metamorphic change has been +greatest, two varieties occur. The first is a hard, compact, white, +fine-grained rock, striped with a few parallel lines of black volcanic +particles, and resembling a sandstone, but which, upon close examination, is +seen to be crystallised throughout, with the cleavages so perfect that they can +be readily measured by the reflecting goniometer. In specimens, where the +change has been less complete, when moistened and examined under a strong lens, +the most interesting gradation can be traced, some of the rounded particles +retaining their proper forms, and others insensibly melting into the +granulo-crystalline paste. The weathered surface of this stone, as is so +frequently the case with ordinary limestones, assumes a brick-red colour. +</p> + +<p> +The second metamorphosed variety is likewise a hard rock, but without any +crystalline structure. It consists of a white, opaque, compact, calcareous +stone, thickly mottled with rounded, though regular, spots of a soft, earthy, +ochraceous substance. This earthy matter is of a pale yellowish- brown colour, +and appears to be a mixture of carbonate of lime with iron; it effervesces with +acids, is infusible, but blackens under the blowpipe, and becomes magnetic. The +rounded form of the minute patches of earthy substance, and the steps in the +progress of their perfect formation, which can be followed in a suit of +specimens, clearly show that they are due either to some power of aggregation +in the earthy particles amongst themselves, or more probably to a strong +attraction between the atoms of the carbonate of line, and consequently to the +segregation of the earthy extraneous matter. I was much interested by this +fact, because I have often seen quartz rocks (for instance, in the Falkland +Islands, and in the lower Silurian strata of the Stiper-stones in Shropshire), +mottled in a precisely analogous manner, with little spots of a white, earthy +substance (earthy feldspar?); and these rocks, there was good reason to +suppose, had undergone the action of heat,—a view which thus receives +confirmation. This spotted structure may possibly afford some indication in +distinguishing those formations of quartz, which owe their present structure to +igneous action, from those produced by the agency of water alone; a source of +doubt, which I should think from my own experience, that most geologists, when +examining arenaceo-quartzose districts must have experienced. +</p> + +<p> +The lowest and most scoriaceous part of the lava, in rolling over the +sedimentary deposit at the bottom of the sea, has caught up large quantities of +calcareous matter, which now forms a snow-white, highly crystalline basis to a +breccia, including small pieces of black, glossy scoriae. A little above this, +where the lime is less abundant, and the lava more compact, numerous little +balls, composed of spicula of calcareous spar, radiating from common centres, +occupy the interstices. In one part of Quail Island, the lime has thus been +crystallised by the heat of the superincumbent lava, where it is only thirteen +feet in thickness; nor had the lava been originally thicker, and since reduced +by degradation, as could be told from the degree of cellularity of its surface. +I have already observed that the sea must have been shallow in which the +calcareous deposit was accumulated. In this case, therefore, the carbonic acid +gas has been retained under a pressure, insignificant compared with that (a +column of water, 1,708 feet in height) originally supposed by Sir James Hall to +be requisite for this end: but since his experiments, it has been discovered +that pressure has less to do with the retention of carbonic acid gas, than the +nature of the circumjacent atmosphere; and hence, as is stated to be the case +by Mr. Faraday, masses of limestone are sometimes fused and crystallised even +in common limekilns. (I am much indebted to Mr. E.W. Brayley in having given me +the following references to papers on this subject: Faraday in the +“Edinburgh New Philosophical Journal” volume 15 page 398; +Gay-Lussac in “Annales de Chem. et Phys.” tome 63 page 219 +translated in the “London and Edinburgh Philosophical Magazine” +volume 10 page 496.) Carbonate of lime can be heated to almost any degree, +according to Faraday, in an atmosphere of carbonic acid gas, without being +decomposed; and Gay-Lussac found that fragments of limestone, placed in a tube +and heated to a degree, not sufficient by itself to cause their decomposition, +yet immediately evolved their carbonic acid, when a stream of common air or +steam was passed over them: Gay-Lussac attributes this to the mechanical +displacement of the nascent carbonic acid gas. The calcareous matter beneath +the lava, and especially that forming the crystalline spicula between the +interstices of the scoriae, although heated in an atmosphere probably composed +chiefly of steam, could not have been subjected to the effects of a passing +stream; and hence it is, perhaps, that they have retained their carbonic acid, +under a small amount of pressure. +</p> + +<p> +The fragments of scoriae, embedded in the crystalline calcareous basis, are of +a jet black colour, with a glossy fracture like pitchstone. Their surfaces, +however, are coated with a layer of a reddish-orange, translucent substance, +which can easily be scratched with a knife; hence they appear as if overlaid by +a thin layer of rosin. Some of the smaller fragments are partially changed +throughout into this substance: a change which appears quite different from +ordinary decomposition. At the Galapagos Archipelago (as will be described in a +future chapter), great beds are formed of volcanic ashes and particles of +scoriae, which have undergone a closely similar change. +</p> + +<h3>THE EXTENT AND HORIZONTALITY OF THE CALCAREOUS STRATUM.</h3> + +<p> +(FIGURE 2: SIGNAL POST HILL. (Section with A low and C high.) +</p> + +<p> +A.—Ancient volcanic rocks. +</p> + +<p> +B.—Calcareous stratum. +</p> + +<p> +C.—Upper basaltic lava.) +</p> + +<p> +The upper line of surface of the calcareous stratum, which is so conspicuous +from being quite white and so nearly horizontal, ranges for miles along the +coast, at the height of about sixty feet above the sea. The sheet of basalt, by +which it is capped, is on an average eighty feet in thickness. Westward of +Porto Praya beyond Red Hill, the white stratum with the superincumbent basalt +is covered up by more recent streams. Northward of Signal Post Hill, I could +follow it with my eye, trending away for several miles along the sea cliffs. +The distance thus observed is about seven miles; but I cannot doubt from its +regularity that it extends much farther. In some ravines at right angles to the +coast, it is seen gently dipping towards the sea, probably with the same +inclination as when deposited round the ancient shores of the island. I found +only one inland section, namely, at the base of the hill marked A, where, at +the height of some hundred feet, this bed was exposed; it here rested on the +usual compact augitic rock associated with wacke, and was covered by the +widespread sheet of modern basaltic lava. Some exceptions occur to the +horizontality of the white stratum: at Quail Island, its upper surface is only +forty feet above the level of the sea; here also the capping of lava is only +between twelve and fifteen feet in thickness; on the other hand, at the +north-east side of Porto Praya harbour, the calcareous stratum, as well as the +rock on which it rests, attain a height above the average level: the inequality +of level in these two cases is not, as I believe, owing to unequal elevation, +but to original irregularities at the bottom of the sea. Of this fact, at Quail +Island, there was clear evidence in the calcareous deposit being in one part of +much greater than the average thickness, and in another part being entirely +absent; in this latter case, the modern basaltic lavas rested directly on those +of more ancient origin. +</p> + +<p> +Under Signal Post Hill, the white stratum dips into the sea in a remarkable +manner. This hill is conical, 450 feet in height, and retains some traces of +having had a crateriform structure; it is composed chiefly of matter erupted +posteriorly to the elevation of the great basaltic plain, but partly of lava of +apparently submarine origin and of considerable antiquity. The surrounding +plain, as well as the eastern flank of this hill, has been worn into steep +precipices, overhanging the sea. In these precipices, the white calcareous +stratum may be seen, at the height of about seventy feet above the beach, +running for some miles both northward and southward of the hill, in a line +appearing to be perfectly horizontal; but for a space of a quarter of a mile +directly under the hill, it dips into the sea and disappears. On the south side +the dip is gradual, on the north side it is more abrupt, as is shown in Figure +2. As neither the calcareous stratum, nor the superincumbent basaltic lava (as +far as the latter can be distinguished from the more modern ejections), appears +to thicken as it dips, I infer that these strata were not originally +accumulated in a trough, the centre of which afterwards became a point of +eruption; but that they have subsequently been disturbed and bent. We may +suppose either that Signal Post Hill subsided after its elevation with the +surrounding country, or that it never was uplifted to the same height with it. +This latter seems to me the most probable alternative, for during the slow and +equable elevation of this portion of the island, the subterranean motive power, +from expending part of its force in repeatedly erupting volcanic matter from +beneath this point, would, it is likely, have less force to uplift it. +Something of the same kind seems to have occurred near Red Hill, for when +tracing upwards the naked streams of lava from near Porto Praya towards the +interior of the island, I was strongly induced to suspect, that since the lava +had flowed, the slope of the land had been slightly modified, either by a small +subsidence near Red Hill, or by that portion of the plain having been uplifted +to a less height during the elevation of the whole area. +</p> + +<h3>THE BASALTIC LAVA, SUPERINCUMBENT ON THE CALCAREOUS DEPOSIT.</h3> + +<p> +This lava is of a pale grey colour, fusing into a black enamel; its fracture is +rather earthy and concretionary; it contains olivine in small grains. The +central parts of the mass are compact, or at most crenulated with a few minute +cavities, and are often columnar. At Quail Island this structure was assumed in +a striking manner; the lava in one part being divided into horizontal laminae, +which became in another part split by vertical fissures into five-sided plates; +and these again, being piled on each other, insensibly became soldered +together, forming fine symmetrical columns. The lower surface of the lava is +vesicular, but sometimes only to the thickness of a few inches; the upper +surface, which is likewise vesicular, is divided into balls, frequently as much +as three feet in diameter, made up of concentric layers. The mass is composed +of more than one stream; its total thickness being, on an average, about eighty +feet: the lower portion has certainly flowed beneath the sea, and probably +likewise the upper portion. The chief part of this lava has flowed from the +central districts, between the hills marked A, B, C, etc., in the woodcut- map. +The surface of the country, near the coast, is level and barren; towards the +interior, the land rises by successive terraces, of which four, when viewed +from a distance, could be distinctly counted. +</p> + +<p> +VOLCANIC ERUPTIONS SUBSEQUENT TO THE ELEVATION OF THE COASTLAND; THE EJECTED +MATTER ASSOCIATED WITH EARTHY LIME. +</p> + +<p> +These recent lavas have proceeded from those scattered, conical, reddish- +coloured hills, which rise abruptly from the plain-country near the coast. I +ascended some of them, but will describe only one, namely, RED HILL, which may +serve as a type of its class, and is remarkable in some especial respects. Its +height is about six hundred feet; it is composed of bright red, highly +scoriaceous rock of a basaltic nature; on one side of its summit there is a +hollow, probably the last remnant of a crater. Several of the other hills of +this class, judging from their external forms, are surmounted by much more +perfect craters. When sailing along the coast, it was evident that a +considerable body of lava had flowed from Red Hill, over a line of cliff about +one hundred and twenty feet in height, into the sea: this line of cliff is +continuous with that forming the coast, and bounding the plain on both sides of +this hill; these streams, therefore, were erupted, after the formation of the +coast-cliffs, from Red Hill, when it must have stood, as it now does, above the +level of the sea. This conclusion accords with the highly scoriaceous condition +of all the rock on it, appearing to be of subaerial formation: and this is +important, as there are some beds of calcareous matter near its summit, which +might, at a hasty glance, have been mistaken for a submarine deposit. These +beds consist of white, earthy, carbonate of lime, extremely friable so as to be +crushed with the least pressure; the most compact specimens not resisting the +strength of the fingers. Some of the masses are as white as quicklime, and +appear absolutely pure; but on examining them with a lens, minute particles of +scoriae can always be seen, and I could find none which, when dissolved in +acids, did not leave a residue of this nature. It is, moreover, difficult to +find a particle of the lime which does not change colour under the blowpipe, +most of them even becoming glazed. The scoriaceous fragments and the calcareous +matter are associated in the most irregular manner, sometimes in obscure beds, +but more generally as a confused breccia, the lime in some parts and the +scoriae in others being most abundant. Sir H. De la Beche has been so kind as +to have some of the purest specimens analysed, with a view to discover, +considering their volcanic origin, whether they contained much magnesia; but +only a small portion was found, such as is present in most limestones. +</p> + +<p> +Fragments of the scoriae embedded in the calcareous mass, when broken, exhibit +many of their cells lined and partly filled with a white, delicate, excessively +fragile, moss-like, or rather conferva-like, reticulation of carbonate of lime. +These fibres, examined under a lens of one-tenth of an inch focal distance, +appear cylindrical; they are rather above one- thousandth of an inch in +diameter; they are either simply branched, or more commonly united into an +irregular mass of network, with the meshes of very unequal sizes and of unequal +numbers of sides. Some of the fibres are thickly covered with extremely minute +spicula, occasionally aggregated into little tuffs; and hence they have a hairy +appearance. These spicula are of the same diameter throughout their length; +they are easily detached, so that the object-glass of the microscope soon +becomes scattered over with them. Within the cells of many fragments of the +scoria, the lime exhibits this fibrous structure, but generally in a less +perfect degree. These cells do not appear to be connected with one another. +There can be no doubt, as will presently be shown, that the lime was erupted, +mingled with the lava in its fluid state, and therefore I have thought it worth +while to describe minutely this curious fibrous structure, of which I know +nothing analogous. From the earthy condition of the fibres, this structure does +not appear to be related to crystallisation. +</p> + +<p> +Other fragments of the scoriaceous rock from this hill, when broken, are often +seen marked with short and irregular white streaks, which are owing to a row of +separate cells being partly, or quite, filled with white calcareous powder. +This structure immediately reminded me of the appearance in badly kneaded +dough, of balls and drawn-out streaks of flour, which have remained unmixed +with the paste; and I cannot doubt that small masses of the lime, in the same +manner remaining unmixed with the fluid lava, have been drawn out when the +whole was in motion. I carefully examined, by trituration and solution in +acids, pieces of the scoriae, taken from within half-an-inch of those cells +which were filled with the calcareous powder, and they did not contain an atom +of free lime. It is obvious that the lava and lime have on a large scale been +very imperfectly mingled; and where small portions of the lime have been +entangled within a piece of the viscid lava, the cause of their now occupying, +in the form of a powder or of a fibrous reticulation, the vesicular cavities, +is, I think, evidently due to the confined gases having most readily expanded +at the points where the incoherent lime rendered the lava less adhesive. +</p> + +<p> +A mile eastward of the town of Praya, there is a steep-sided gorge, about one +hundred and fifty yards in width, cutting through the basaltic plain and +underlying beds, but since filled up by a stream of more modern lava. This lava +is dark grey, and in most parts compact and rudely columnar; but at a little +distance from the coast, it includes in an irregular manner a brecciated mass +of red scoriae mingled with a considerable quantity of white, friable, and in +some parts, nearly pure earthy lime, like that on the summit of Red Hill. This +lava, with its entangled lime, has certainly flowed in the form of a regular +stream; and, judging from the shape of the gorge, towards which the drainage of +the country (feeble though it now be) still is directed, and from the +appearance of the bed of loose water-worn blocks with their interstices +unfilled, like those in the bed of a torrent, on which the lava rests, we may +conclude that the stream was of subaerial origin. I was unable to trace it to +its source, but, from its direction, it seemed to have come from Signal Post +Hill, distant one mile and a quarter, which, like Red Hill, has been a point of +eruption subsequent to the elevation of the great basaltic plain. It accords +with this view, that I found on Signal Post Hill, a mass of earthy, calcareous +matter of the same nature, mingled with scoriae. I may here observe that part +of the calcareous matter forming the horizontal sedimentary bed, especially the +finer matter with which the embedded fragments of rock are whitewashed, has +probably been derived from similar volcanic eruptions, as well as from +triturated organic remains: the underlying, ancient, crystalline rocks, also, +are associated with much carbonate of lime, filling amygdaloidal cavities, and +forming irregular masses, the nature of which latter I was unable to +understand. +</p> + +<p> +Considering the abundance of earthy lime near the summit of Red Hill, a +volcanic cone six hundred feet in height, of subaerial +growth,—considering the intimate manner in which minute particles and +large masses of scoriae are embedded in the masses of nearly pure lime, and on +the other hand, the manner in which small kernels and streaks of the calcareous +powder are included in solid pieces of the scoriae,—considering, also, +the similar occurrence of lime and scoriae within a stream of lava, also +supposed, with good reason, to have been of modern subaerial origin, and to +have flowed from a hill, where earthy lime also occurs: I think, considering +these facts, there can be no doubt that the lime has been erupted, mingled with +the molten lava. I am not aware that any similar case has been described: it +appears to me an interesting one, inasmuch as most geologists must have +speculated on the probable effects of a volcanic focus, bursting through +deep-seated beds of different mineralogical composition. The great abundance of +free silex in the trachytes of some countries (as described by Beudant in +Hungary, and by P. Scrope in the Panza Islands), perhaps solves the inquiry +with respect to deep-seated beds of quartz; and we probably here see it +answered, where the volcanic action has invaded subjacent masses of limestone. +One is naturally led to conjecture in what state the now earthy carbonate of +lime existed, when ejected with the intensely heated lava: from the extreme +cellularity of the scoriae on Red Hill, the pressure cannot have been great, +and as most volcanic eruptions are accompanied by the emission of large +quantities of steam and other gases, we here have the most favourable +conditions, according to the views at present entertained by chemists, for the +expulsion of the carbonic acid. (Whilst deep beneath the surface, the carbonate +of lime was, I presume, in a fluid state. Hutton, it is known, thought that all +amygdaloids were produced by drops of molten limestone floating in the trap, +like oil in water: this no doubt is erroneous, but if the matter forming the +summit of Red Hill had been cooled under the pressure of a moderately deep sea, +or within the walls of a dike, we should, in all probability, have had a trap +rock associated with large masses of compact, crystalline, calcareous spar, +which, according to the views entertained by many geologists, would have been +wrongly attributed to subsequent infiltration.) Has the slow re- absorption of +this gas, it may be asked, given to the lime in the cells of the lava, that +peculiar fibrous structure, like that of an efflorescing salt? Finally, I may +remark on the great contrast in appearance between this earthy lime, which must +have been heated in a free atmosphere of steam and other gases, while the +white, crystalline, calcareous spar, produced by a single thin sheet of lava +(as at Quail Island) rolling over similar earthy lime and the debris of organic +remains, at the bottom of a shallow sea. +</p> + +<h3>SIGNAL POST HILL.</h3> + +<p> +This hill has already been several times mentioned, especially with reference +to the remarkable manner in which the white calcareous stratum, in other parts +so horizontal (Figure 2), dips under it into the sea. It has a broad summit, +with obscure traces of a crateriform structure, and is composed of basaltic +rocks (Of these, one common variety is remarkable for being full of small +fragments of a dark jasper-red earthy mineral, which, when examined carefully, +shows an indistinct cleavage; the little fragments are elongated in form, are +soft, are magnetic before and after being heated, and fuse with difficulty into +a dull enamel. This mineral is evidently closely related to the oxides of iron, +but I cannot ascertain what it exactly is. The rock containing this mineral is +crenulated with small angular cavities, which are lined and filled with +yellowish crystals of carbonate of lime.), some compact, others highly cellular +with inclined beds of loose scoriae, of which some are associated with earthy +lime. Like Red Hill, it has been the source of eruptions, subsequently to the +elevation of the surrounding basaltic plain; but unlike that hill, it has +undergone considerable denudation, and has been the seat of volcanic action at +a remote period, when beneath the sea. I judge of this latter circumstance from +finding on its inland flank the last remains of three small points of eruption. +These points are composed of glossy scoriae, cemented by crystalline calcareous +spar, exactly like the great submarine calcareous deposit, where the heated +lava has rolled over it: their demolished state can, I think, be explained only +by the denuding action of the waves of the sea. I was guided to the first +orifice by observing a sheet of lava, about two hundred yards square, with +steepish sides, superimposed on the basaltic plain with no adjoining hillock, +whence it could have been erupted; and the only trace of a crater which I was +able to discover, consisted of some inclined beds of scoriae at one of its +corners. At the distance of fifty yards from a second level-topped patch of +lava, but of much smaller size, I found an irregular circular group of masses +of cemented, scoriaceous breccia, about six feet in height, which doubtless had +once formed the point of eruption. The third orifice is now marked only by an +irregular circle of cemented scoriae, about four yards in diameter, and rising +in its highest point scarcely three feet above the level of the plain, the +surface of which, close all round, exhibits its usual appearance: here we have +a horizontal basal section of a volcanic spiracle, which, together with all its +ejected matter, has been almost totally obliterated. +</p> + +<p> +The stream of lava, which fills the narrow gorge eastward of the town of Praya, +judging from its course, seems, as before remarked, to have come from Signal +Post Hill, and to have flowed over the plain, after its elevation (The sides of +this gorge, where the upper basaltic stratum is intersected, are almost +perpendicular. The lava, which has since filled it up, is attached to these +sides, almost as firmly as a dike is to its walls. In most cases, where a +stream of lava has flowed down a valley, it is bounded on each side by loose +scoriaceous masses.): the same observation applies to a stream (possibly part +of the same one) capping the sea cliffs, a little eastward of the gorge. When I +endeavoured to follow these streams over the stony level plain, which is almost +destitute of soil and vegetation, I was much surprised to find, that although +composed of hard basaltic matter, and not having been exposed to marine +denudation, all distant traces of them soon became utterly lost. But I have +since observed at the Galapagos Archipelago, that it is often impossible to +follow even great deluges of quite recent lava across older streams, except by +the size of the bushes growing on them, or by the comparative states of +glossiness of their surfaces,—characters which a short lapse of time +would be sufficient quite to obscure. I may remark, that in a level country, +with a dry climate, and with the wind blowing always in one direction (as at +the Cape de Verde Archipelago), the effects of atmospheric degradation are +probably much greater than would at first be expected; for soil in this case +accumulates only in a few protected hollows, and being blown in one direction, +it is always travelling towards the sea in the form of the finest dust, leaving +the surface of the rocks bare, and exposed to the full effects of renewed +meteoric action. +</p> + +<h3>INLAND HILLS OF MORE ANCIENT VOLCANIC ROCKS.</h3> + +<p> +These hills are laid down by eye, and marked as A, B, C, etc., in Map 1. They +are related in mineralogical composition, and are probably directly continuous +with the lowest rocks exposed on the coast. These hills, viewed from a +distance, appear as if they had once formed part of an irregular tableland, and +from their corresponding structure and composition this probably has been the +case. They have flat, slightly inclined summits, and are, on an average, about +six hundred feet in height; they present their steepest slope towards the +interior of the island, from which point they radiate outwards, and are +separated from each other by broad and deep valleys, through which the great +streams of lava, forming the coast-plains, have descended. Their inner and +steeper escarpments are ranged in an irregular curve, which rudely follows the +line of the shore, two or three miles inland from it. I ascended a few of these +hills, and from others, which I was able to examine with a telescope, I +obtained specimens, through the kindness of Mr. Kent, the assistant-surgeon of +the “Beagle”; although by these means I am acquainted with only a +part of the range, five or six miles in length, yet I scarcely hesitate, from +their uniform structure, to affirm that they are parts of one great formation, +stretching round much of the circumference of the island. +</p> + +<p> +The upper and lower strata of these hills differ greatly in composition. The +upper are basaltic, generally compact, but sometimes scoriaceous and +amygdaloidal, with associated masses of wacke: where the basalt is compact, it +is either fine-grained or very coarsely crystallised; in the latter case it +passes into an augitic rock, containing much olivine; the olivine is either +colourless, or of the usual yellow and dull reddish shades. On some of the +hills, beds of calcareous matter, both in an earthy and in a crystalline form, +including fragments of glossy scoriae, are associated with the basaltic strata. +These strata differ from the streams of basaltic lava forming the coast-plains, +only in being more compact, and in the crystals of augite, and in the grains of +olivine being of much greater size;—characters which, together with the +appearance of the associated calcareous beds, induce me to believe that they +are of submarine formation. +</p> + +<p> +Some considerable masses of wacke, which are associated with these basaltic +strata, and which likewise occur in the basal series on the coast, especially +at Quail Island, are curious. They consist of a pale yellowish- green +argillaceous substance, of a crumbling texture when dry, but unctuous when +moist: in its purest form, it is of a beautiful green tint, with translucent +edges, and occasionally with obscure traces of an original cleavage. Under the +blowpipe it fuses very readily into a dark grey, and sometimes even black bead, +which is slightly magnetic. From these characters, I naturally thought that it +was one of the pale species, decomposed, of the genus augite;—a +conclusion supported by the unaltered rock being full of large separate +crystals of black augite, and of balls and irregular streaks of dark grey +augitic rock. As the basalt ordinarily consists of augite, and of olivine often +tarnished and of a dull red colour, I was led to examine the stages of +decomposition of this latter mineral, and I found, to my surprise, that I could +trace a nearly perfect gradation from unaltered olivine to the green wacke. +Part of the same grain under the blowpipe would in some instances behave like +olivine, its colour being only slightly changed, and part would give a black +magnetic bead. Hence I can have no doubt that the greenish wacke originally +existed as olivine; but great chemical changes must have been effected during +the act of decomposition thus to have altered a very hard, transparent, +infusible mineral, into a soft, unctuous, easily melted, argillaceous +substance. (D’Aubuisson “Traite de Geognosie” tome 2 page 569 +mentions, on the authority of M. Marcel de Serres, masses of green earth near +Montpellier, which are supposed to be due to the decomposition of olivine. I do +not, however, find, that the action of this mineral under the blowpipe being +entirely altered, as it becomes decomposed, has been noticed; and the knowledge +of this fact is important, as at first it appears highly improbable that a +hard, transparent, refractory mineral should be changed into a soft, easily +fused clay, like this of St. Jago. I shall hereafter describe a green +substance, forming threads within the cells of some vesicular basaltic rocks in +Van Diemen’s Land, which behave under the blowpipe like the green wacke +of St. Jago; but its occurrence in cylindrical threads, shows it cannot have +resulted from the decomposition of olivine, a mineral always existing in the +form of grains or crystals.) +</p> + +<p> +The basal strata of these hills, as well as some neighbouring, separate, bare, +rounded hillocks, consist of compact, fine-grained, non-crystalline (or so +slightly as scarcely to be perceptible), ferruginous, feldspathic rocks, and +generally in a state of semi-decomposition. Their fracture is exceedingly +irregular, and splintery; yet small fragments are often very tough. They +contain much ferruginous matter, either in the form of minute grains with a +metallic lustre, or of brown hair-like threads: the rock in this latter case +assuming a pseudo-brecciated structure. These rocks sometimes contain mica and +veins of agate. Their rusty brown or yellowish colour is partly due to the +oxides of iron, but chiefly to innumerable, microscopically minute, black +specks, which, when a fragment is heated, are easily fused, and evidently are +either hornblende or augite. These rocks, therefore, although at first +appearing like baked clay or some altered sedimentary deposit, contain all the +essential ingredients of trachyte; from which they differ only in not being +harsh, and in not containing crystals of glassy feldspar. As is so often the +case with trachytic formation, no stratification is here apparent. A person +would not readily believe that these rocks could have flowed as lava; yet at +St. Helena there are well-characterised streams (as will be described in an +ensuing chapter) of nearly similar composition. Amidst the hillocks composed of +these rocks, I found in three places, smooth conical hills of phonolite, +abounding with fine crystals of glassy feldspar, and with needles of +hornblende. These cones of phonolite, I believe, bear the same relation to the +surrounding feldspathic strata which some masses of coarsely crystallised +augitic rock, in another part of the island, bear to the surrounding basalt, +namely, that both have been injected. The rocks of a feldspathic nature being +anterior in origin to the basaltic strata, which cap them, as well as to the +basaltic streams of the coast-plains, accords with the usual order of +succession of these two grand divisions of the volcanic series. +</p> + +<p> +The strata of most of these hills in the upper part, where alone the planes of +division are distinguishable, are inclined at a small angle from the interior +of the island towards the sea-coast. The inclination is not the same in each +hill; in that marked A it is less than in B, D, or E; in C the strata are +scarcely deflected from a horizontal plane, and in F (as far as I could judge +without ascending it) they are slightly inclined in a reverse direction, that +is, inwards and towards the centre of the island. Notwithstanding these +differences of inclination, their correspondence in external form, and in the +composition both of their upper and lower parts,- -their relative position in +one curved line, with their steepest sides turned inwards,—all seem to +show that they originally formed parts of one platform; which platform, as +before remarked, probably extended round a considerable portion of the +circumference of the island. The upper strata certainly flowed as lava, and +probably beneath the sea, as perhaps did the lower feldspathic masses: how then +come these strata to hold their present position, and whence were they erupted? +</p> + +<p> +In the centre of the island there are lofty mountains, but they are separated +from the steep inland flanks of these hills by a wide space of lower country: +the interior mountains, moreover, seem to have been the source of those great +streams of basaltic lava which, contracting as they pass between the bases of +the hills in question, expand into the coast- plains. (I saw very little of the +inland parts of the island. Near the village of St. Domingo, there are +magnificent cliffs of rather coarsely crystallised basaltic lava. Following the +little stream in this valley, about a mile above the village, the base of the +great cliff was formed of a compact fine-grained basalt, conformably covered by +a bed of pebbles. Near Fuentes, I met with pap-formed hills of the compact +feldspathic series of rocks.) Round the shores of St. Helena there is a rudely +formed ring of basaltic rocks, and at Mauritius there are remnants of another +such a ring round part, if not round the whole, of the island; here again the +same question immediately occurs, how came these masses to hold their present +position, and whence were they erupted? The same answer, whatever it may be, +probably applies in these three cases; and in a future chapter we shall recur +to this subject. +</p> + +<h3>VALLEYS NEAR THE COAST.</h3> + +<p> +These are broad, very flat, and generally bounded by low cliff-formed sides. +Portions of the basaltic plain are sometimes nearly or quite isolated by them; +of which fact, the space on which the town of Praya stands offers an instance. +The great valley west of the town has its bottom filled up to a depth of more +than twenty feet by well-rounded pebbles, which in some parts are firmly +cemented together by white calcareous matter. There can be no doubt, from the +form of these valleys, that they were scooped out by the waves of the sea, +during that equable elevation of the land, of which the horizontal calcareous +deposit, with its existing species of marine remains, gives evidence. +Considering how well shells have been preserved in this stratum, it is singular +that I could not find even a single small fragment of shell in the conglomerate +at the bottom of the valleys. The bed of pebbles in the valley west of the town +is intersected by a second valley joining it as a tributary, but even this +valley appears much too wide and flat-bottomed to have been formed by the small +quantity of water, which falls only during one short wet season; for at other +times of the year these valleys are absolutely dry. +</p> + +<h3>RECENT CONGLOMERATE.</h3> + +<p> +On the shores of Quail Island, I found fragments of brick, bolts of iron, +pebbles, and large fragments of basalt, united by a scanty base of impure +calcareous matter into a firm conglomerate. To show how exceedingly firm this +recent conglomerate is, I may mention, that I endeavoured with a heavy +geological hammer to knock out a thick bolt of iron, which was embedded a +little above low-water mark, but was quite unable to succeed. +</p> + + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap02"></a>CHAPTER II.<br /> +FERNANDO NORONHA; TERCEIRA; TAHITI, ETC.</h2> + +<p> +FERNANDO NORONHA. Precipitous hill of phonolite. +</p> + +<p> +TERCEIRA. Trachytic rocks: their singular decomposition by steam of high +temperature. +</p> + +<p> +TAHITI. Passage from wacke into trap; singular volcanic rock with the vesicles +half-filled with mesotype. +</p> + +<p> +MAURITIUS. Proofs of its recent elevation. Structure of its more ancient +mountains; similarity with St. Jago. +</p> + +<p> +ST. PAUL’S ROCKS. Not of volcanic origin. Their singular mineralogical +composition. +</p> + +<h3>FERNANDO NORONHA.</h3> + +<p> +During our short visit at this and the four following islands, I observed very +little worthy of description. Fernando Noronha is situated in the Atlantic +Ocean, in latitude 3 degrees 50 minutes S., and 230 miles distant from the +coast of South America. It consists of several islets, together nine miles in +length by three in breadth. The whole seems to be of volcanic origin; although +there is no appearance of any crater, or of any one central eminence. The most +remarkable feature is a hill 1,000 feet high, of which the upper 400 feet +consist of a precipitous, singularly shaped pinnacle, formed of columnar +phonolite, containing numerous crystals of glassy feldspar, and a few needles +of hornblende. From the highest accessible point of this hill, I could +distinguish in different parts of the group several other conical hills, +apparently of the same nature. At St. Helena there are similar, great, conical, +protuberant masses of phonolite, nearly one thousand feet in height, which have +been formed by the injection of fluid feldspathic lava into yielding strata. If +this hill has had, as is probable, a similar origin, denudation has been here +effected on an enormous scale. Near the base of this hill, I observed beds of +white tuff, intersected by numerous dikes, some of amygdaloidal basalt and +others of trachyte; and beds of slaty phonolite with the planes of cleavage +directed N.W. and S.E. Parts of this rock, where the crystals were scanty, +closely resembled common clay-slate, altered by the contact of a trap-dike. The +lamination of rocks, which undoubtedly have once been fluid, appears to me a +subject well deserving attention. On the beach there were numerous fragments of +compact basalt, of which rock a distant facade of columns seemed to be formed. +</p> + +<h3>TERCEIRA IN THE AZORES.</h3> + +<p> +The central parts of this island consist of irregularly rounded mountains of no +great elevation, composed of trachyte, which closely resembles in general +character the trachyte of Ascension, presently to be described. This formation +is in many parts overlaid, in the usual order of superposition, by streams of +basaltic lava, which near the coast compose nearly the whole surface. The +course which these streams have followed from their craters, can often be +followed by the eye. The town of Angra is overlooked by a crateriform hill +(Mount Brazil), entirely built of thin strata of fine-grained, harsh, +brown-coloured tuff. The upper beds are seen to overlap the basaltic streams on +which the town stands. This hill is almost identical in structure and +composition with numerous crateriformed hills in the Galapagos Archipelago. +</p> + +<h3>EFFECTS OF STEAM ON THE TRACHYTIC ROCKS.</h3> + +<p> +In the central part of the island there is a spot, where steam is constantly +issuing in jets from the bottom of a small ravine-like hollow, which has no +exit, and which abuts against a range of trachytic mountains. The steam is +emitted from several irregular fissures: it is scentless, soon blackens iron, +and is of much too high temperature to be endured by the hand. The manner in +which the solid trachyte is changed on the borders of these orifices is +curious: first, the base becomes earthy, with red freckles evidently due to the +oxidation of particles of iron; then it becomes soft; and lastly, even the +crystals of glassy feldspar yield to the dissolving agent. After the mass is +converted into clay, the oxide of iron seems to be entirely removed from some +parts, which are left perfectly white, whilst in other neighbouring parts, +which are of the brightest red colour, it seems to be deposited in greater +quantity; some other masses are marbled with two distinct colours. Portions of +the white clay, now that they are dry, cannot be distinguished by the eye from +the finest prepared chalk; and when placed between the teeth they feel equally +soft-grained; the inhabitants use this substance for white-washing their +houses. The cause of the iron being dissolved in one part, and close by being +again deposited, is obscure; but the fact has been observed in several other +places. (Spallanzani, Dolomieu, and Hoffman have described similar cases in the +Italian volcanic islands. Dolomieu says the iron at the Panza Islands is +redeposited in the form of veins (page 86 “Memoire sur les Isles +Ponces”). These authors likewise believe that the steam deposits silica: +it is now experimentally known that vapour of a high temperature is able to +dissolve silica.) In some half-decayed specimens, I found small, globular +aggregations of yellow hyalite, resembling gum-arabic, which no doubt had been +deposited by the steam. +</p> + +<p> +As there is no escape for the rain-water, which trickles down the sides of the +ravine-like hollow, whence the steam issues, it must all percolate downwards +through the fissures at its bottom. Some of the inhabitants informed me that it +was on record that flames (some luminous appearance?) had originally proceeded +from these cracks, and that the flames had been succeeded by the steam; but I +was not able to ascertain how long this was ago, or anything certain on the +subject. When viewing the spot, I imagined that the injection of a large mass +of rock. like the cone of phonolite at Fernando Noronha, in a semi-fluid state, +by arching the surface might have caused a wedge-shaped hollow with cracks at +the bottom, and that the rain- water percolating to the neighbourhood of the +heated mass, would during many succeeding years be driven back in the form of +steam. +</p> + +<h3>TAHITI (OTAHEITE).</h3> + +<p> +I visited only a part of the north-western side of this island, and this part +is entirely composed of volcanic rocks. Near the coast there are several +varieties of basalt, some abounding with large crystals of augite and tarnished +olivine, others compact and earthy,—some slightly vesicular, and others +occasionally amygdaloidal. These rocks are generally much decomposed, and to my +surprise, I found in several sections that it was impossible to distinguish, +even approximately, the line of separation between the decayed lava and the +alternating beds of tuff. Since the specimens have become dry, it is rather +more easy to distinguish the decomposed igneous rocks from the sedimentary +tuffs. This gradation in character between rocks having such widely different +origins, may I think be explained by the yielding under pressure of the +softened sides of the vesicular cavities, which in many volcanic rocks occupy a +large proportion of their bulk. As the vesicles generally increase in size and +number in the upper parts of a stream of lava, so would the effects of their +compression increase; the yielding, moreover, of each lower vesicle must tend +to disturb all the softened matter above it. Hence we might expect to trace a +perfect gradation from an unaltered crystalline rock to one in which all the +particles (although originally forming part of the same solid mass) had +undergone mechanical displacement; and such particles could hardly be +distinguished from others of similar composition, which had been deposited as +sediment. As lavas are sometimes laminated in their upper parts even horizontal +lines, appearing like those of aqueous deposition, could not in all cases be +relied on as a criterion of sedimentary origin. From these considerations it is +not surprising that formerly many geologists believed in real transitions from +aqueous deposits, through wacke, into igneous traps. +</p> + +<p> +In the valley of Tia-auru, the commonest rocks are basalts with much olivine, +and in some cases almost composed of large crystals of augite. I picked up some +specimens, with much glassy feldspar, approaching in character to trachyte. +There were also many large blocks of vesicular basalt, with the cavities +beautifully lined with chabasie (?), and radiating bundles of mesotype. Some of +these specimens presented a curious appearance, owing to a number of the +vesicles being half filled up with a white, soft, earthy mesotypic mineral, +which intumesced under the blowpipe in a remarkable manner. As the upper +surfaces in all the half-filled cells are exactly parallel, it is evident that +this substance has sunk to the bottom of each cell from its weight. Sometimes, +however, it entirely fills the cells. Other cells are either quite filled, or +lined, with small crystals, apparently of chabasie; these crystals, also, +frequently line the upper half of the cells partly filled with the earthy +mineral, as well as the upper surface of this substance itself, in which case +the two minerals appear to blend into each other. I have never seen any other +amygdaloid with the cells half filled in the manner here described; and it is +difficult to imagine the causes which determined the earthy mineral to sink +from its gravity to the bottom of the cells, and the crystalline mineral to +adhere in a coating of equal thickness round the sides of the cells. +(MacCulloch, however, has described and given a plate of (“Geolog. +Trans.” 1st series volume 4 page 225) a trap rock, with cavities filled +up horizontally with quartz and chalcedony. The upper halves of these cavities +are often filled by layers, which follow each irregularity of the surface, and +by little depending stalactites of the same siliceous substances.) +</p> + +<p> +The basic strata on the sides of the valley are gently inclined seaward, and I +nowhere observed any sign of disturbance; the strata are separated from each +other by thick, compact beds of conglomerate, in which the fragments are large, +some being rounded, but most angular. From the character of these beds, from +the compact and crystalline condition of most of the lavas, and from the nature +of the infiltrated minerals, I was led to conjecture that they had originally +flowed beneath the sea. This conclusion agrees with the fact that the Rev. W. +Ellis found marine remains at a considerable height, which he believes were +interstratified with volcanic matter; as is likewise described to be the case +by Messrs. Tyerman and Bennett at Huaheine, an island in this same archipelago. +Mr. Stutchbury also discovered near the summit of one of the loftiest mountains +of Tahiti, at the height of several thousand feet, a stratum of semi-fossil +coral. None of these remains have been specifically examined. On the coast, +where masses of coral-rock would have afforded the clearest evidence, I looked +in vain for any signs of recent elevation. For references to the above +authorities, and for more detailed reasons for not believing that Tahiti has +been recently elevated, I must refer to the “Structure and Distribution +of Coral-Reefs.” +</p> + +<h3>MAURITIUS.</h3> + +<p> +Approaching this island on the northern or north-western side, a curved chain +of bold mountains, surmounted by rugged pinnacles, is seen to rise from a +smooth border of cultivated land, which gently slopes down to the coast. At the +first glance, one is tempted to believe that the sea lately reached the base of +these mountains, and upon examination, this view, at least with respect to the +inferior parts of the border, is found to be perfectly correct. Several authors +have described masses of upraised coral- rock round the greater part of the +circumference of the island. (Captain Carmichael, in Hooker’s “Bot. +Misc.” volume 2 page 301. Captain Lloyd has lately, in the +“Proceedings of the Geological Society” (volume 3 page 317), +described carefully some of these masses. In the “Voyage a l’Isle +de France, par un Officier du Roi,” many interesting facts are given on +this subject. Consult also “Voyage aux Quatre Isles d’Afrique, par +M. Bory St. Vincent.”) Between Tamarin Bay and the Great Black River I +observed, in company with Captain Lloyd, two hillocks of coral-rock, formed in +their lower part of hard calcareous sandstone, and in their upper of great +blocks, slightly aggregated, of Astraea and Madrepora, and of fragments of +basalt; they were divided into beds dipping seaward, in one case at an angle of +8 degrees, and in the other at 18 degrees; they had a water-worn appearance, +and they rose abruptly from a smooth surface, strewed with rolled debris of +organic remains, to a height of about twenty feet. The Officier du Roi, in his +most interesting tour in 1768 round the island, has described masses of +upraised coral-rocks, still retaining that moat-like structure (see my +“Coral Reefs”) which is characteristic of the living reefs. On the +coast northward of Port Louis, I found the lava concealed for a considerable +space inland by a conglomerate of corals and shells, like those on the beach, +but in parts consolidated by red ferruginous matter. M. Bory St. Vincent has +described similar calcareous beds over nearly the whole of the plain of +Pamplemousses. Near Port Louis, when turning over some large stones, which lay +in the bed of a stream at the head of a protected creek, and at the height of +some yards above the level of spring tides, I found several shells of serpula +still adhering to their under sides. +</p> + +<p> +The jagged mountains near Port Louis rise to a height of between two and three +thousand feet; they consist of strata of basalt, obscurely separated from each +other by firmly aggregated beds of fragmentary matter; and they are intersected +by a few vertical dikes. The basalt in some parts abounds with large crystals +of augite and olivine, and is generally compact. The interior of the island +forms a plain, raised probably about a thousand feet above the level of the +sea, and composed of streams of lava which have flowed round and between the +rugged basaltic mountains. These more recent lavas are also basaltic, but less +compact, and some of them abound with feldspar, so that they even fuse into a +pale coloured glass. On the banks of the Great River, a section is exposed +nearly five hundred feet deep, worn through numerous thin sheets of the lava of +this series, which are separated from each other by beds of scoriae. They seem +to have been of subaerial formation, and to have flowed from several points of +eruption on the central platform, of which the Piton du Milieu is said to be +the principal one. There are also several volcanic cones, apparently of this +modern period, round the circumference of the island, especially at the +northern end, where they form separate islets. +</p> + +<p> +The mountains composed of the more compact and crystalline basalt, form the +main skeleton of the island. M. Bailly (“Voyage aux Terres +Australes” tome 1 page 54.) states that they all “se developpent +autour d’elle comme une ceinture d’immenses remparts, toutes +affectant une pente plus ou moins enclinee vers le rivage de la mer; tandis, au +contraire, que vers le centre de l’ile elles presentent une coupe +abrupte, et souvent taillee a pic. Toutes ces montagnes sont formees de couches +paralleles inclinees du centre de l’ile vers la mer.” These +statements have been disputed, though not in detail, by M. Quoy, in the voyage +of Freycinet. As far as my limited means of observation went, I found them +perfectly correct. (M. Lesson, in his account of this island, in the +“Voyage of the ‘Coquille’,” seems to follow M. +Bailly’s views.) The mountains on the N.W. side of the island, which I +examined, namely, La Pouce, Peter Botts, Corps de Garde, Les Mamelles, and +apparently another farther southward, have precisely the external shape and +stratification described by M. Bailly. They form about a quarter of his girdle +of ramparts. Although these mountains now stand quite detached, being separated +from each other by breaches, even several miles in width, through which deluges +of lava have flowed from the interior of the island; nevertheless, seeing their +close general similarity, one must feel convinced that they originally formed +parts of one continuous mass. Judging from the beautiful map of the Mauritius, +published by the Admiralty from a French MS., there is a range of mountains (M. +Bamboo) on the opposite side of the island, which correspond in height, +relative position, and external form, with those just described. Whether the +girdle was ever complete may well be doubted; but from M. Bailly’s +statements, and my own observations, it may be safely concluded that mountains +with precipitous inland flanks, and composed of strata dipping outwards, once +extended round a considerable portion of the circumference of the island. The +ring appears to have been oval and of vast size; its shorter axis, measured +across from the inner sides of the mountains near Port Louis and those near +Grand Port, being no less than thirteen geographical miles in length. M. Bailly +boldly supposes that this enormous gulf, which has since been filled up to a +great extent by streams of modern lava, was formed by the sinking in of the +whole upper part of one great volcano. +</p> + +<p> +It is singular in how many respects those portions of St. Jago and of Mauritius +which I visited agree in their geological history. At both islands, mountains +of similar external form, stratification, and (at least in their upper beds) +composition, follow in a curved chain the coast-line. These mountains in each +case appear originally to have formed parts of one continuous mass. The +basaltic strata of which they are composed, from their compact and crystalline +structure, seem, when contrasted with the neighbouring basaltic streams of +subaerial formation, to have flowed beneath the pressure of the sea, and to +have been subsequently elevated. We may suppose that the wide breaches between +the mountains were in both cases worn by the waves, during their gradual +elevation—of which process, within recent times, there is abundant +evidence on the coast-land of both islands. At both, vast streams of more +recent basaltic lavas have flowed from the interior of the island, round and +between the ancient basaltic hills; at both, moreover, recent cones of eruption +are scattered around the circumference of the island; but at neither have +eruptions taken place within the period of history. As remarked in the last +chapter, it is probable that these ancient basaltic mountains, which resemble +(at least in many respects) the basal and disturbed remnants of two gigantic +volcanoes, owe their present form, structure, and position, to the action of +similar causes. +</p> + +<h3>ST. PAUL’S ROCKS.</h3> + +<p> +This small island is situated in the Atlantic Ocean, nearly one degree north of +the equator, and 540 miles distant from South America, in 29 degrees 15 minutes +west longitude. Its highest point is scarcely fifty feet above the level of the +sea; its outline is irregular, and its entire circumference barely +three-quarters of a mile. This little point of rock rises abruptly out of the +ocean; and, except on its western side, soundings were not obtained, even at +the short distance of a quarter of a mile from its shore. It is not of volcanic +origin; and this circumstance, which is the most remarkable point in its +history (as will hereafter be referred to), properly ought to exclude it from +the present volume. It is composed of rocks, unlike any which I have met with, +and which I cannot characterise by any name, and must therefore describe. +</p> + +<p> +The simplest, and one of the most abundant kinds, is a very compact, heavy, +greenish-black rock, having an angular, irregular fracture, with some points +just hard enough to scratch glass, and infusible. This variety passes into +others of paler green tints, less hard, but with a more crystalline fracture, +and translucent on their edges; and these are fusible into a green enamel. +Several other varieties are chiefly characterised by containing innumerable +threads of dark-green serpentine, and by having calcareous matter in their +interstices. These rocks have an obscure, concretionary structure, and are full +of variously coloured angular pseudo fragments. These angular pseudo fragments +consist of the first-described dark green rock, of a brown softer kind, of +serpentine, and of a yellowish harsh stone, which, perhaps, is related to +serpentine rock. There are other vesicular, calcareo-ferruginous, soft stones. +There is no distinct stratification, but parts are imperfectly laminated; and +the whole abounds with innumerable veins, and vein-like masses, both small and +large. Of these vein-like masses, some calcareous ones, which contain minute +fragments of shells, are clearly of subsequent origin to the others. +</p> + +<h3>A GLOSSY INCRUSTATION.</h3> + +<p> +Extensive portions of these rocks are coated by a layer of a glossy polished +substance, with a pearly lustre and of a greyish white colour; it follows all +the inequalities of the surface, to which it is firmly attached. When examined +with a lens, it is found to consist of numerous exceedingly thin layers, their +aggregate thickness being about the tenth of an inch. It is considerably harder +than calcareous spar, but can be scratched with a knife; under the blowpipe it +scales off, decrepitates, slightly blackens, emits a fetid odour, and becomes +strongly alkaline: it does not effervesce in acids. (In my +“Journal” I have described this substance; I then believed that it +was an impure phosphate of lime.) I presume this substance has been deposited +by water draining from the birds’ dung, with which the rocks are covered. +At Ascension, near a cavity in the rocks which was filled with a laminated mass +of infiltrated birds’ dung, I found some irregularly formed, +stalactitical masses of apparently the same nature. These masses, when broken, +had an earthy texture; but on their outsides, and especially at their +extremities, they were formed of a pearly substance, generally in little +globules, like the enamel of teeth, but more translucent, and so hard as just +to scratch plate-glass. This substance slightly blackens under the blowpipe, +emits a bad smell, then becomes quite white, swelling a little, and fuses into +a dull white enamel; it does not become alkaline; nor does it effervesce in +acids. The whole mass had a collapsed appearance, as if in the formation of the +hard glossy crust the whole had shrunk much. At the Abrolhos Islands on the +coast of Brazil, where also there is much birds’ dung, I found a great +quantity of a brown, arborescent substance adhering to some trap-rock. In its +arborescent form, this substance singularly resembles some of the branched +species of Nullipora. Under the blowpipe, it behaves like the specimens from +Ascension; but it is less hard and glossy, and the surface has not the shrunk +appearance. +</p> + + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap03"></a>CHAPTER III.<br /> +ASCENSION.</h2> + +<p> +Basaltic lavas. Numerous craters truncated on the same side. Singular structure +of volcanic bombs. Aeriform explosions. Ejected granitic fragments. Trachytic +rocks. Singular veins. Jasper, its manner of formation. Concretions in +pumiceous tuff. Calcareous deposits and frondescent incrustations on the coast. +Remarkable laminated beds, alternating with, and passing into, obsidian. Origin +of obsidian. Lamination of volcanic rocks. +</p> + +<h3>(MAP 2: THE ISLAND OF ASCENSION.)</h3> + +<p> +This island is situated in the Atlantic Ocean, in latitude 8 degrees S., +longitude 14 degrees W. It has the form of an irregular triangle (see Map 2), +each side being about six miles in length. Its highest point is 2,870 feet +(“Geographical Journal” volume 5 page 243.) above the level of the +sea. The whole is volcanic, and, from the absence of proofs to the contrary, I +believe of subaerial origin. The fundamental rock is everywhere of a pale +colour, generally compact, and of a feldspathic nature. In the S.E. portion of +the island, where the highest land is situated, well characterised trachyte, +and other congenerous rocks of that varying family, occur. Nearly the entire +circumference is covered up by black and rugged streams of basaltic lava, with +here and there a hill or single point of rock (one of which near the sea-coast, +north of the Fort, is only two or three yards across) of the trachyte still +remaining exposed. +</p> + +<h3>BASALTIC ROCKS.</h3> + +<p> +The overlying basaltic lava is in some parts extremely vesicular, in others +little so; it is of a black colour, but sometimes contains crystals of glassy +feldspar, and seldom much olivine. These streams appear to have possessed +singularly little fluidity; their side walls and lower ends being very steep, +and even as much as between twenty and thirty feet in height. Their surface is +extraordinarily rugged, and from a short distance appears as if studded with +small craters. These projections consist of broad, irregularly conical, +hillocks, traversed by fissures, and composed of the same unequally scoriaceous +basalt with the surrounding streams, but having an obscure tendency to a +columnar structure; they rise to a height between ten and thirty feet above the +general surface, and have been formed, as I presume, by the heaping up of the +viscid lava at points of greater resistance. At the base of several of these +hillocks, and occasionally likewise on more level parts, solid ribs, composed +of angulo-globular masses of basalt, resembling in size and outline arched +sewers or gutters of brickwork, but not being hollow, project between two or +three feet above the surface of the streams; what their origin may have been, I +do not know. Many of the superficial fragments from these basaltic streams +present singularly convoluted forms; and some specimens could hardly be +distinguished from logs of dark-coloured wood without their bark. +</p> + +<p> +Many of the basaltic streams can be traced, either to points of eruption at the +base of the great central mass of trachyte, or to separate, conical, +red-coloured hills, which are scattered over the northern and western borders +of the island. Standing on the central eminence, I counted between twenty and +thirty of these cones of eruption. The greater number of them had their +truncated summits cut off obliquely, and they all sloped towards the S.E., +whence the trade-wind blows. (M. Lesson in the “Zoology of the Voyage of +the ‘Coquille’” page 490 has observed this fact. Mr. Hennah +(“Geolog. Proceedings” 1835 page 189) further remarks that the most +extensive beds of ashes at Ascension invariably occur on the leeward side of +the island.) This structure no doubt has been caused by the ejected fragments +and ashes being always blown, during eruptions, in greater quantity towards one +side than towards the other. M. Moreau de Jonnes has made a similar observation +with respect to the volcanic orifices in the West Indian Islands. +</p> + +<h3>VOLCANIC BOMBS.</h3> + +<p> +(FIGURE 3: FRAGMENT OF A SPHERICAL VOLCANIC BOMB, with the interior parts +coarsely cellular, coated by a concentric layer of compact lava, and this again +by a crust of finely cellular rock. +</p> + +<p> +FIGURE 4: VOLCANIC BOMB OF OBSIDIAN FROM AUSTRALIA. The upper figure gives a +front view; the lower a side view of the same object.) +</p> + +<p> +These occur in great numbers strewed on the ground, and some of them lie at +considerable distances from any points of eruption. They vary in size from that +of an apple to that of a man’s body; they are either spherical or +pear-shaped, or with the hinder part (corresponding to the tail of a comet) +irregular, studded with projecting points, and even concave. Their surfaces are +rough, and fissured with branching cracks; their internal structure is either +irregularly scoriaceous and compact, or it presents a symmetrical and very +curious appearance. An irregular segment of a bomb of this latter kind, of +which I found several, is accurately represented in Figure 3. Its size was +about that of a man’s head. The whole interior is coarsely cellular; the +cells averaging in diameter about the tenth of an inch; but nearer the outside +they gradually decrease in size. This part is succeeded by a well-defined shell +of compact lava, having a nearly uniform thickness of about the third of an +inch; and the shell is overlaid by a somewhat thicker coating of finely +cellular lava (the cells varying from the fiftieth to the hundredth of an inch +in diameter), which forms the external surface: the line separating the shell +of compact lava from the outer scoriaceous crust is distinctly defined. This +structure is very simply explained, if we suppose a mass of viscid, scoriaceous +matter, to be projected with a rapid, rotatory motion through the air; for +whilst the external crust, from cooling, became solidified (in the state we now +see it), the centrifugal force, by relieving the pressure in the interior parts +of the bomb, would allow the heated vapours to expand their cells; but these +being driven by the same force against the already-hardened crust, would +become, the nearer they were to this part, smaller and smaller or less +expanded, until they became packed into a solid, concentric shell. As we know +that chips from a grindstone (Nichol “Architecture of the +Heavens.”) can be flirted off, when made to revolve with sufficient +velocity, we need not doubt that the centrifugal force would have power to +modify the structure of a softened bomb, in the manner here supposed. +Geologists have remarked, that the external form of a bomb at once bespeaks the +history of its aerial course, and few now see that the internal structure can +speak, with almost equal plainness, of its rotatory movement. +</p> + +<p> +M. Bory St. Vincent (“Voyage aux Quatre Isles d’Afrique” tome +1 page 222.) has described some balls of lava from the Isle of Bourbon, which +have a closely similar structure. His explanation, however (if I understand it +rightly), is very different from that which I have given; for he supposes that +they have rolled, like snowballs, down the sides of the crater. M. Beudant +(“Voyage en Hongrie” tome 2 page 214.), also, has described some +singular little balls of obsidian, never more than six or eight inches in +diameter, which he found strewed on the surface of the ground: their form is +always oval; sometimes they are much swollen in the middle, and even +spindle-shaped: their surface is regularly marked with concentric ridges and +furrows, all of which on the same ball are at right angles to one axis: their +interior is compact and glassy. M. Beudant supposes that masses of lava, when +soft, were shot into the air, with a rotatory movement round the same axis, and +that the form and superficial ridges of the bombs were thus produced. Sir +Thomas Mitchell has given me what at first appears to be the half of a much +flattened oval ball of obsidian; it has a singular artificial-like appearance, +which is well represented (of the natural size) in Figure 4. It was found in +its present state, on a great sandy plain between the rivers Darling and +Murray, in Australia, and at the distance of several hundred miles from any +known volcanic region. It seems to have been embedded in some reddish tufaceous +matter; and may have been transported either by the aborigines or by natural +means. The external saucer consists of compact obsidian, of a bottle-green +colour, and is filled with finely cellular black lava, much less transparent +and glassy than the obsidian. The external surface is marked with four or five +not quite perfect ridges, which are represented rather too distinctly in Figure +4. Here, then, we have the external structure described by M. Beudant, and the +internal cellular condition of the bombs from Ascension. The lip of the saucer +is slightly concave, exactly like the margin of a soup-plate, and its inner +edge overlaps a little the central cellular lava. This structure is so +symmetrical round the entire circumference, that one is forced to suppose that +the bomb burst during its rotatory course, before being quite solidified, and +that the lip and edges were thus slightly modified and turned inwards. It may +be remarked that the superficial ridges are in planes, at right angles to an +axis, transverse to the longer axis of the flattened oval: to explain this +circumstance, we may suppose that when the bomb burst, the axis of rotation +changed. +</p> + +<h3>AERIFORM EXPLOSIONS.</h3> + +<p> +The flanks of Green Mountain and the surrounding country are covered by a great +mass, some hundred feet in thickness, of loose fragments. The lower beds +generally consist of fine-grained, slightly consolidated tuffs (Some of this +peperino, or tuff, is sufficiently hard not to be broken by the greatest force +of the fingers.), and the upper beds of great loose fragments, with alternating +finer beds. (On the northern side of the Green Mountain a thin seam, about an +inch in thickness, of compact oxide of iron, extends over a considerable area; +it lies conformably in the lower part of the stratified mass of ashes and +fragments. This substance is of a reddish- brown colour, with an almost +metallic lustre; it is not magnetic, but becomes so after having been heated +under the blowpipe, by which it is blackened and partly fused. This seam of +compact stone, by intercepting the little rain-water which falls on the island, +gives rise to a small dripping spring, first discovered by Dampier. It is the +only fresh water on the island, so that the possibility of its being inhabited +has entirely depended on the occurrence of this ferruginous layer.) One white +ribbon- like layer of decomposed, pumiceous breccia, was curiously bent into +deep unbroken curves, beneath each of the large fragments in the superincumbent +stratum. From the relative position of these beds, I presume that a narrow- +mouthed crater, standing nearly in the position of Green Mountain, like a great +air-gun, shot forth, before its final extinction, this vast accumulation of +loose matter. Subsequently to this event, considerable dislocations have taken +place, and an oval circus has been formed by subsidence. This sunken space lies +at the north-eastern foot of Green Mountain, and is well represented in Map 2. +Its longer axis, which is connected with a N.E. and S.W. line of fissure, is +three-fifths of a nautical mile in length; its sides are nearly perpendicular, +except in one spot, and about four hundred feet in height; they consist, in the +lower part, of a pale basalt with feldspar, and in the upper part, of the tuff +and loose ejected fragments; the bottom is smooth and level, and under almost +any other climate a deep lake would have been formed here. From the thickness +of the bed of loose fragments, with which the surrounding country is covered, +the amount of aeriform matter necessary for their projection must have been +enormous; hence we may suppose it probable that after the explosions vast +subterranean caverns were left, and that the falling in of the roof of one of +these produced the hollow here described. At the Galapagos Archipelago, pits of +a similar character, but of a much smaller size, frequently occur at the bases +of small cones of eruption. +</p> + +<h3>EJECTED GRANITIC FRAGMENTS.</h3> + +<p> +In the neighbourhood of Green Mountain, fragments of extraneous rock are not +unfrequently found embedded in the midst of masses of scoriae. Lieutenant +Evans, to whose kindness I am indebted for much information, gave me several +specimens, and I found others myself. They nearly all have a granitic +structure, are brittle, harsh to the touch, and apparently of altered colours. +</p> + +<p> +FIRST, a white syenite, streaked and mottled with red; it consists of well- +crystallised feldspar, numerous grains of quartz, and brilliant, though small, +crystals of hornblende. The feldspar and hornblende in this and the succeeding +cases have been determined by the reflecting goniometer, and the quartz by its +action under the blowpipe. The feldspar in these ejected fragments, like the +glassy kind in the trachyte, is from its cleavage a potash-feldspar. +</p> + +<p> +SECONDLY, a brick-red mass of feldspar, quartz, and small dark patches of a +decayed mineral; one minute particle of which I was able to ascertain, by its +cleavage, to be hornblende. +</p> + +<p> +THIRDLY, a mass of confusedly crystallised white feldspar, with little nests of +a dark-coloured mineral, often carious, externally rounded, having a glossy +fracture, but no distinct cleavage: from comparison with the second specimen, I +have no doubt that it is fused hornblende. +</p> + +<p> +FOURTHLY, a rock, which at first appears a simple aggregation of distinct and +large-sized crystals of dusty-coloured Labrador feldspar (Professor Miller has +been so kind as to examine this mineral. He obtained two good cleavages of 86 +degrees 30 minutes and 86 degrees 50 minutes. The mean of several, which I +made, was 86 degrees 30 minutes. Professor Miller states that these crystals, +when reduced to a fine powder, are soluble in hydrochloric acid, leaving some +undissolved silex behind; the addition of oxalate of ammonia gives a copious +precipitate of lime. He further remarks, that according to Von Kobell, +anorthite (a mineral occurring in the ejected fragments at Mount Somma) is +always white and transparent, so that if this be the case, these crystals from +Ascension must be considered as Labrador feldspar. Professor Miller adds, that +he has seen an account, in Erdmann’s “Journal fur tecnische +Chemie,” of a mineral ejected from a volcano which had the external +characters of Labrador feldspar, but differed in the analysis from that given +by mineralogists of this mineral: the author attributed this difference to an +error in the analysis of Labrador feldspar, which is very old.); but in their +interstices there is some white granular feldspar, abundant scales of mica, a +little altered hornblende, and, as I believe, no quartz. I have described these +fragments in detail, because it is rare to find granitic rocks ejected from +volcanoes with their MINERALS UNCHANGED, as is the case with the first +specimen, and partially with the second. (Daubeny, in his work on Volcanoes +page 386, remarks that this is the case; and Humboldt, in his “Personal +Narrative” volume 1 page 236, says “In general, the masses of known +primitive rocks, I mean those which perfectly resemble our granites, gneiss, +and mica-slate, are very rare in lavas: the substances we generally denote by +the name of granite, thrown out by Vesuvius, are mixtures of nepheline, mica, +and pyroxene.”) One other large fragment, found in another spot, is +deserving of notice; it is a conglomerate, containing small fragments of +granitic, cellular, and jaspery rocks, and of hornstone porphyries, embedded in +a base of wacke, threaded by numerous thin layers of a concretionary pitchstone +passing into obsidian. These layers are parallel, slightly tortuous, and short; +they thin out at their ends, and resemble in form the layers of quartz in +gneiss. It is probable that these small embedded fragments were not separately +ejected, but were entangled in a fluid volcanic rock, allied to obsidian; and +we shall presently see that several varieties of this latter series of rock +assume a laminated structure. +</p> + +<h3>TRACHYTIC SERIES OF ROCKS.</h3> + +<p> +Those occupy the more elevated and central, and likewise the south-eastern, +parts of the island. The trachyte is generally of a pale brown colour, stained +with small darker patches; it contains broken and bent crystals of glassy +feldspar, grains of specular iron, and black microscopical points, which +latter, from being easily fused, and then becoming magnetic, I presume are +hornblende. The greater number of the hills, however, are composed of a quite +white, friable stone, appearing like a trachytic tuff. Obsidian, hornstone, and +several kinds of laminated feldspathic rocks, are associated with the trachyte. +There is no distinct stratification; nor could I distinguish a crateriform +structure in any of the hills of this series. Considerable dislocations have +taken place; and many fissures in these rocks are yet left open, or are only +partially filled with loose fragments. Within the space (This space is nearly +included by a line sweeping round Green Mountain, and joining the hills, called +the Weather Port Signal, Holyhead, and that denominated (improperly in a +geological sense) “the Crater of an old volcano.”), mainly formed +of trachyte, some basaltic streams have burst forth; and not far from the +summit of Green Mountain, there is one stream of quite black, vesicular basalt, +containing minute crystals of glassy feldspar, which have a rounded appearance. +</p> + +<p> +The soft white stone above mentioned is remarkable from its singular +resemblance, when viewed in mass, to a sedimentary tuff: it was long before I +could persuade myself that such was not its origin; and other geologists have +been perplexed by closely similar formations in trachytic regions. In two +cases, this white earthy stone formed isolated hills; in a third, it was +associated with columnar and laminated trachyte; but I was unable to trace an +actual junction. It contains numerous crystals of glassy feldspar and black +microscopical specks, and is marked with small darker patches, exactly as in +the surrounding trachyte. Its basis, however, when viewed under the microscope, +is generally quite earthy; but sometimes it exhibits a decidedly crystalline +structure. On the hill marked “Crater of an old volcano,” it passes +into a pale greenish-grey variety, differing only in its colour, and in not +being so earthy; the passage was in one case effected insensibly; in another, +it was formed by numerous, rounded and angular, masses of the greenish variety, +being embedded in the white variety;—in this latter case, the appearance +was very much like that of a sedimentary deposit, torn up and abraded during +the deposition of a subsequent stratum. Both these varieties are traversed by +innumerable tortuous veins (presently to be described), which are totally +unlike injected dikes, or indeed any other veins which I have ever seen. Both +varieties include a few scattered fragments, large and small, of dark- coloured +scoriaceous rocks, the cells of some of which are partially filled with the +white earthy stone; they likewise include some huge blocks of a cellular +porphyry. (The porphyry is dark coloured; it contains numerous, often +fractured, crystals of white opaque feldspar, also decomposing crystals of +oxide of iron; its vesicles include masses of delicate, hair- like, crystals, +apparently of analcime.) These fragments project from the weathered surface, +and perfectly resemble fragments embedded in a true sedimentary tuff. But as it +is known that extraneous fragments of cellular rock are sometimes included in +columnar trachyte, in phonolite (D’Aubuisson “Traite de +Geognosie” tome 2 page 548.), and in other compact lavas, this +circumstance is not any real argument for the sedimentary origin of the white +earthy stone. (Dr. Daubeny on Volcanoes, page 180 seems to have been led to +believe that certain trachytic formations of Ischia and of the Puy de Dome, +which closely resemble these of Ascension, were of sedimentary origin, chiefly +from the frequent presence in them “of scoriform portions, different in +colour from the matrix.” Dr. Daubeny adds, that on the other hand, +Brocchi, and other eminent geologists, have considered these beds as earthy +varieties of trachyte; he considers the subject deserving of further +attention.) The insensible passage of the greenish variety into the white one, +and likewise the more abrupt passage by fragments of the former being embedded +in the latter, might result from slight differences in the composition of the +same mass of molten stone, and from the abrading action of one such part still +fluid on another part already solidified. The curiously formed veins have, I +believe, been formed by siliceous matter being subsequently segregated. But my +chief reason for believing that these soft earthy stones, with their extraneous +fragments, are not of sedimentary origin, is the extreme improbability of +crystals of feldspar, black microscopical specks, and small stains of a darker +colour occurring in the same proportional numbers in an aqueous deposit, and in +masses of solid trachyte. Moreover, as I have remarked, the microscope +occasionally reveals a crystalline structure in the apparently earthy basis. On +the other hand, the partial decomposition of such great masses of trachyte, +forming whole mountains, is undoubtedly a circumstance of not easy explanation. +</p> + +<h3>VEINS IN THE EARTHY TRACHYTIC MASSES.</h3> + +<p> +These veins are extraordinarily numerous, intersecting in the most complicated +manner both coloured varieties of the earthy trachyte: they are best seen on +the flanks of the “Crater of the old volcano.” They contain +crystals of glassy feldspar, black microscopical specks and little dark stains, +precisely as in the surrounding rock; but the basis is very different, being +exceedingly hard, compact, somewhat brittle, and of rather less easy +fusibility. The veins vary much, and suddenly, from the tenth of an inch to one +inch in thickness; they often thin out, not only on their edges, but in their +central parts, thus leaving round, irregular apertures; their surfaces are +rugged. They are inclined at every possible angle with the horizon, or are +horizontal; they are generally curvilinear, and often interbranch one with +another. From their hardness they withstand weathering, and projecting two or +three feet above the ground, they occasionally extend some yards in length; +these plate-like veins, when struck, emit a sound, almost like that of a drum, +and they may be distinctly seen to vibrate; their fragments, which are strewed +on the ground, clatter like pieces of iron when knocked against each other. +They often assume the most singular forms; I saw a pedestal of the earthy +trachyte, covered by a hemispherical portion of a vein, like a great umbrella, +sufficiently large to shelter two persons. I have never met with, or seen +described, any veins like these; but in form they resemble the ferruginous +seams, due to some process of segregation, occurring not uncommonly in +sandstones,—for instance, in the New Red sandstone of England. Numerous +veins of jasper and of siliceous sinter, occurring on the summit of this same +hill, show that there has been some abundant source of silica, and as these +plate-like veins differ from the trachyte only in their greater hardness, +brittleness, and less easy fusibility, it appears probable that their origin is +due to the segregation or infiltration of siliceous matter, in the same manner +as happens with the oxides of iron in many sedimentary rocks. +</p> + +<h3>SILICEOUS SINTER AND JASPER.</h3> + +<p> +The siliceous sinter is either quite white, of little specific gravity, and +with a somewhat pearly fracture, passing into pinkish pearl quartz; or it is +yellowish white, with a harsh fracture, and it then contains an earthy powder +in small cavities. Both varieties occur, either in large irregular masses in +the altered trachyte, or in seams included in broad, vertical, tortuous, +irregular veins of a compact, harsh stone of a dull red colour, appearing like +a sandstone. This stone, however, is only altered trachyte; and a nearly +similar variety, but often honeycombed, sometimes adheres to the projecting +plate-like veins, described in the last paragraph. The jasper is of an ochre +yellow or red colour; it occurs in large irregular masses, and sometimes in +veins, both in the altered trachyte and in an associated mass of scoriaceous +basalt. The cells of the scoriaceous basalt are lined or filled with fine, +concentric layers of chalcedony, coated and studded with bright-red oxide of +iron. In this rock, especially in the rather more compact parts, irregular +angular patches of the red jasper are included, the edges of which insensibly +blend into the surrounding mass; other patches occur having an intermediate +character between perfect jasper and the ferruginous, decomposed, basaltic +base. In these patches, and likewise in the large vein-like masses of jasper, +there occur little rounded cavities, of exactly the same size and form with the +air-cells, which in the scoriaceous basalt are filled and lined with layers of +chalcedony. Small fragments of the jasper, examined under the microscope, seem +to resemble the chalcedony with its colouring matter not separated into layers, +but mingled in the siliceous paste, together with some impurities. I can +understand these facts,—namely, the blending of the jasper into the +semi-decomposed basalt,—its occurrence in angular patches, which clearly +do not occupy pre-existing hollows in the rock,—and its containing little +vesicles filled with chalcedony, like those in the scoriaceous lava,—only +on the supposition that a fluid, probably the same fluid which deposited the +chalcedony in the air-cells, removed in those parts where there were no +cavities, the ingredients of the basaltic rock, and left in their place silica +and iron, and thus produced the jasper. In some specimens of silicified wood, I +have observed, that in the same manner as in the basalt, the solid parts were +converted into a dark-coloured homogeneous stone, whereas the cavities formed +by the larger sap-vessels (which may be compared with the air-vesicles in the +basaltic lava) and other irregular hollows, apparently produced by decay, were +filled with concentric layers of chalcedony; in this case, there can be little +doubt that the same fluid deposited the homogeneous base and the chalcedonic +layers. After these considerations, I cannot doubt but that the jasper of +Ascension may be viewed as a volcanic rock silicified, in precisely the same +sense as this term is applied to wood, when silicified; we are equally ignorant +of the means by which every atom of wood, whilst in a perfect state, is removed +and replaced by atoms of silica, as we are of the means by which the +constituent parts of a volcanic rock could be thus acted on. (Beudant +“Voyage en Hongrie” tome 3 pages 502, 504 describes kidney-shaped +masses of jasper-opal, which either blend into the surrounding trachytic +conglomerate, or are embedded in it like chalk-flints; and he compares them +with the fragments of opalised wood, which are abundant in this same formation. +Beudant, however, appears to have viewed the process of their formation rather +as one of simple infiltration than of molecular exchange; but the presence of a +concretion, wholly different from the surrounding matter, if not formed in a +pre-existing hollow, clearly seems to me to require, either a molecular or +mechanical displacement of the atoms, which occupied the space afterwards +filled by it. The jasper-opal of Hungary passes into chalcedony, and therefore +in this case, as in that of Ascension, jasper seems to be intimately related in +origin with chalcedony.) I was led to the careful examination of these rocks, +and to the conclusion here given, from having heard the Rev. Professor Henslow +express a similar opinion, regarding the origin in trap-rocks of many +chalcedonies and agates. Siliceous deposits seem to be very general, if not of +universal occurrence, in partially decomposed trachytic tuffs (Beudant +“Voyage Min.” tome 3 page 507 enumerates cases in Hungary, Germany, +Central France, Italy, Greece, and Mexico.); and as these hills, according to +the view above given, consist of trachyte softened and altered in situ, the +presence of free silica in this case may be added as one more instance to the +list. +</p> + +<h3>CONCRETIONS IN PUMICEOUS TUFF.</h3> + +<p> +The hill, marked in Map 2 “Crater of an old volcano,” has no claims +to this appellation, which I could discover, except in being surmounted by a +circular, very shallow, saucer-like summit, nearly half a mile in diameter. +This hollow has been nearly filled up with many successive sheets of ashes and +scoriae, of different colours, and slightly consolidated. Each successive +saucer-shaped layer crops out all round the margin, forming so many rings of +various colours, and giving to the hill a fantastic appearance. The outer ring +is broad, and of a white colour; hence it resembles a course round which horses +have been exercised, and has received the name of the Devil’s Riding +School, by which it is most generally known. These successive layers of ashes +must have fallen over the whole surrounding country, but they have all been +blown away except in this one hollow, in which probably moisture accumulated, +either during an extraordinary year when rain fell, or during the storms often +accompanying volcanic eruptions. One of the layers of a pinkish colour, and +chiefly derived from small, decomposed fragments of pumice, is remarkable, from +containing numerous concretions. These are generally spherical, from half an +inch to three inches in diameter; but they are occasionally cylindrical, like +those of iron-pyrites in the chalk of Europe. They consist of a very tough, +compact, pale-brown stone, with a smooth and even fracture. They are divided +into concentric layers by thin white partitions, resembling the external +superficies; six or eight of such layers are distinctly defined near the +outside; but those towards the inside generally become indistinct, and blend +into a homogeneous mass. I presume that these concentric layers were formed by +the shrinking of the concretion, as it became compact. The interior part is +generally fissured by minute cracks or septaria, which are lined, both by +black, metallic, and by other white and crystalline specks, the nature of which +I was unable to ascertain. Some of the larger concretions consist of a mere +spherical shell, filled with slightly consolidated ashes. The concretions +contain a small proportion of carbonate of lime: a fragment placed under the +blowpipe decrepitates, then whitens and fuses into a blebby enamel, but does +not become caustic. The surrounding ashes do not contain any carbonate of lime; +hence the concretions have probably been formed, as is so often the case, by +the aggregation of this substance. I have not met with any account of similar +concretions; and considering their great toughness and compactness, their +occurrence in a bed, which probably has been subjected only to atmospheric +moisture, is remarkable. +</p> + +<h3>FORMATION OF CALCAREOUS ROCKS ON THE SEA-COAST.</h3> + +<p> +On several of the sea-beaches, there are immense accumulations of small, +well-rounded particles of shells and corals, of white, yellowish, and pink +colours, interspersed with a few volcanic particles. At the depth of a few +feet, these are found cemented together into stone, of which the softer +varieties are used for building; there are other varieties, both coarse and +fine-grained, too hard for this purpose: and I saw one mass divided into even +layers half an inch in thickness, which were so compact that when struck with a +hammer they rang like flint. It is believed by the inhabitants, that the +particles become united in the course of a single year. The union is effected +by calcareous matter; and in the most compact varieties, each rounded particle +of shell and volcanic rock can be distinctly seen to be enveloped in a husk of +pellucid carbonate of lime. Extremely few perfect shells are embedded in these +agglutinated masses; and I have examined even a large fragment under a +microscope, without being able to discover the least vestige of striae or other +marks of external form: this shows how long each particle must have been rolled +about, before its turn came to be embedded and cemented. (The eggs of the +turtle being buried by the parent, sometimes become enclosed in the solid rock. +Mr. Lyell has given a figure (“Principles of Geology” book 3 +chapter 17) of some eggs, containing the bones of young turtles, found thus +entombed.) One of the most compact varieties, when placed in acid, was entirely +dissolved, with the exception of some flocculent animal matter; its specific +gravity was 2.63. The specific gravity of ordinary limestone varies from 2.6 to +2.75; pure Carrara marble was found by Sir H. De la Beche to be 2.7. +(“Researches in Theoretical Geology” page 12.) It is remarkable +that these rocks of Ascension, formed close to the surface, should be nearly as +compact as marble, which has undergone the action of heat and pressure in the +plutonic regions. +</p> + +<p> +The great accumulation of loose calcareous particles, lying on the beach near +the Settlement, commences in the month of October, moving towards the S.W., +which, as I was informed by Lieutenant Evans, is caused by a change in the +prevailing direction of the currents. At this period the tidal rocks, at the +S.W. end of the beach, where the calcareous sand is accumulating, and round +which the currents sweep, become gradually coated with a calcareous +incrustation, half an inch in thickness. It is quite white, compact, with some +parts slightly spathose, and is firmly attached to the rock. After a short time +it gradually disappears, being either redissolved, when the water is less +charged with lime, or more probably is mechanically abraded. Lieutenant Evans +has observed these facts, during the six years he has resided at Ascension. The +incrustation varies in thickness in different years: in 1831 it was unusually +thick. When I was there in July, there was no remnant of the incrustation; but +on a point of basalt, from which the quarrymen had lately removed a mass of the +calcareous freestone, the incrustation was perfectly preserved. Considering the +position of the tidal-rocks, and the period at which they become coated, there +can be no doubt that the movement and disturbance of the vast accumulation of +calcareous particles, many of them being partially agglutinated together, cause +the waves of the sea to be so highly charged with carbonate of lime, that they +deposit it on the first objects against which they impinge. I have been +informed by Lieutenant Holland, R.N., that this incrustation is formed on many +parts of the coast, on most of which, I believe, there are likewise great +masses of comminuted shells. +</p> + +<h3>A FRONDESCENT CALCAREOUS INCRUSTATION.</h3> + +<p> +(FIGURE 5. AN INCRUSTATION OF CALCAREOUS AND ANIMAL MATTER, coating the +tidal-rocks at Ascension.) +</p> + +<p> +In many respects this is a singular deposit; it coats throughout the year the +tidal volcanic rocks, that project from the beaches composed of broken shells. +Its general appearance is well represented in Figure 5; but the fronds or +discs, of which it is composed, are generally so closely crowded together as to +touch. These fronds have their sinuous edges finely crenulated, and they +project over their pedestals or supports; their upper surfaces are either +slightly concave, or slightly convex; they are highly polished, and of a dark +grey or jet black colour; their form is irregular, generally circular, and from +the tenth of an inch to one inch and a half in diameter; their thickness, or +amount of their projection from the rock on which they stand, varies much, +about a quarter of an inch being perhaps most usual. The fronds occasionally +become more and more convex, until they pass into botryoidal masses with their +summits fissured; when in this state, they are glossy and of an intense black, +so as to resemble some fused metallic substance. I have shown the incrustation, +both in this latter and in its ordinary state to several geologists, but not +one could conjecture its origin, except that perhaps it was of volcanic nature! +</p> + +<p> +The substance forming the fronds has a very compact and often almost +crystalline fracture; the edges being translucent, and hard enough easily to +scratch calcareous spar. Under the blowpipe it immediately becomes white, and +emits a strong animal odour, like that from fresh shells. It is chiefly +composed of carbonate of lime; when placed in muriatic acid it froths much, +leaving a residue of sulphate of lime, and of an oxide of iron, together with a +black powder, which is not soluble in heated acids. This latter substance seems +to be carbonaceous, and is evidently the colouring matter. The sulphate of lime +is extraneous, and occurs in distinct, excessively minute, lamellar plates, +studded on the surface of the fronds, and embedded between the fine layers of +which they are composed; when a fragment is heated in the blowpipe, these +lamellae are immediately rendered visible. The original outline of the fronds +may often be traced, either to a minute particle of shell fixed in a crevice of +the rock, or to several cemented together; these first become deeply corroded, +by the dissolving power of the waves, into sharp ridges, and then are coated +with successive layers of the glossy, grey, calcareous incrustation. The +inequalities of the primary support affect the outline of every successive +layer, in the same manner as may often be seen in bezoar-stones, when an object +like a nail forms the centre of aggregation. The crenulated edges, however, of +the frond appear to be due to the corroding power of the surf on its own +deposit, alternating with fresh depositions. On some smooth basaltic rocks on +the coast of St. Jago, I found an exceedingly thin layer of brown calcareous +matter, which under a lens presented a miniature likeness of the crenulated and +polished fronds of Ascension; in this case a basis was not afforded by any +projecting extraneous particles. Although the incrustation at Ascension is +persistent throughout the year; yet from the abraded appearance of some parts, +and from the fresh appearance of other parts, the whole seems to undergo a +round of decay and renovation, due probably to changes in the form of the +shifting beach, and consequently in the action of the breakers: hence probably +it is, that the incrustation never acquires a great thickness. Considering the +position of the encrusted rocks in the midst of the calcareous beach, together +with its composition, I think there can be no doubt that its origin is due to +the dissolution and subsequent deposition of the matter composing the rounded +particles of shells and corals. (The selenite, as I have remarked is +extraneous, and must have been derived from the sea-water. It is an interesting +circumstance thus to find the waves of the ocean, sufficiently charged with +sulphate of lime, to deposit it on the rocks, against which they dash every +tide. Dr. Webster has described (“Voyage of the +‘Chanticleer’” volume 2 page 319) beds of gypsum and salt, as +much as two feet in thickness, left by the evaporation of the spray on the +rocks on the windward coast. Beautiful stalactites of selenite, resembling in +form those of carbonate of lime, are formed near these beds. Amorphous masses +of gypsum, also, occur in caverns in the interior of the island; and at Cross +Hill (an old crater) I saw a considerable quantity of salt oozing from a pile +of scoriae. In these latter cases, the salt and gypsum appear to be volcanic +products.) From this source it derives its animal matter, which is evidently +the colouring principle. The nature of the deposit, in its incipient stage, can +often be well seen upon a fragment of white shell, when jammed between two of +the fronds; it then appears exactly like the thinnest wash of a pale grey +varnish. Its darkness varies a little, but the jet blackness of some of the +fronds and of the botryoidal masses seems due to the translucency of the +successive grey layers. There is, however, this singular circumstance, that +when deposited on the under side of ledges of rock or in fissures, it appears +always to be of a pale, pearly grey colour, even when of considerable +thickness: hence one is led to suppose, that an abundance of light is necessary +to the development of the dark colour, in the same manner as seems to be the +case with the upper and exposed surfaces of the shells of living mollusca, +which are always dark, compared with their under surfaces and with the parts +habitually covered by the mantle of the animal. In this circumstance,—in +the immediate loss of colour and in the odour emitted under the +blowpipe,—in the degree of hardness and translucency of the +edges,—and in the beautiful polish of the surface (From the fact +described in my “Journal of Researches” of a coating of oxide of +iron, deposited by a streamlet on the rocks in its bed (like a nearly similar +coating at the great cataracts of the Orinoco and Nile), becoming finely +polished where the surf acts, I presume that the surf in this instance, also, +is the polishing agent.), rivalling when in a fresh state that of the finest +Oliva, there is a striking analogy between this inorganic incrustation and the +shells of living molluscous animals. (In the section descriptive of St. +Paul’s Rocks, I have described a glossy, pearly substance, which coats +the rocks, and an allied stalactitical incrustation from Ascension, the crust +of which resembles the enamel of teeth, but is hard enough to scratch +plate-glass. Both these substances contain animal matter, and seem to have been +derived from water in filtering through birds’ dung.) This appears to me +to be an interesting physiological fact. (Mr. Horner and Sir David Brewster +have described “Philosophical Transactions” 1836 page 65 a singular +“artificial substance, resembling shell.” It is deposited in fine, +transparent, highly polished, brown- coloured laminae, possessing peculiar +optical properties, on the inside of a vessel, in which cloth, first prepared +with glue and then with lime, is made to revolve rapidly in water. It is much +softer, more transparent, and contains more animal matter, than the natural +incrustation at Ascension; but we here again see the strong tendency which +carbonate of lime and animal matter evince to form a solid substance allied to +shell.) +</p> + +<h3>SINGULAR LAMINATED BEDS ALTERNATING WITH AND PASSING INTO OBSIDIAN.</h3> + +<p> +These beds occur within the trachytic district, at the western base of Green +Mountain, under which they dip at a high inclination. They are only partially +exposed, being covered up by modern ejections; from this cause, I was unable to +trace their junction with the trachyte, or to discover whether they had flowed +as a stream of lava, or had been injected amidst the overlying strata. There +are three principal beds of obsidian, of which the thickest forms the base of +the section. The alternating stony layers appear to me eminently curious, and +shall be first described, and afterwards their passage into the obsidian. They +have an extremely diversified appearance; five principal varieties may be +noticed, but these insensibly blend into each other by endless gradations. +</p> + +<h3>FIRST.</h3> + +<p> +A pale grey, irregularly and coarsely laminated (This term is open to some +misinterpretation, as it may be applied both to rocks divided into laminae of +exactly the same composition, and to layers firmly attached to each other, with +no fissile tendency, but composed of different minerals, or of different shades +of colour. The term “laminated,” in this chapter, is applied in +these latter senses; where a homogeneous rock splits, as in the former sense, +in a given direction, like clay-slate, I have used the term +“fissile.”), harsh-feeling rock, resembling clay-slate which has +been in contact with a trap-dike, and with a fracture of about the same degree +of crystalline structure. This rock, as well as the following varieties, easily +fuses into a pale glass. The greater part is honeycombed with irregular, +angular, cavities, so that the whole has a curious appearance, and some +fragments resemble in a remarkable manner silicified logs of decayed wood. This +variety, especially where more compact, is often marked with thin whitish +streaks, which are either straight or wrap round, one behind the other, the +elongated carious hollows. +</p> + +<h3>SECONDLY.</h3> + +<p> +A bluish grey or pale brown, compact, heavy, homogeneous stone, with an +angular, uneven, earthy fracture; viewed, however, under a lens of high power, +the fracture is seen to be distinctly crystalline, and even separate minerals +can be distinguished. +</p> + +<h3>THIRDLY.</h3> + +<p> +A stone of the same kind with the last, but streaked with numerous, parallel, +slightly tortuous, white lines of the thickness of hairs. These white lines are +more crystalline than the parts between them; and the stone splits along them: +they frequently expand into exceedingly thin cavities, which are often only +just perceptible with a lens. The matter forming the white lines becomes better +crystallised in these cavities, and Professor Miller was fortunate enough, +after several trials, to ascertain that the white crystals, which are the +largest, were of quartz (Professor Miller informs me that the crystals which he +measured had the faces P, z, m of the figure (147) given by Haidinger in his +Translation of Mohs; and he adds, that it is remarkable, that none of them had +the slightest trace of faces r of the regular six-sided prism.), and that the +minute green transparent needles were augite, or, as they would more generally +be called, diopside: besides these crystals, there are some minute, dark specks +without a trace of crystalline, and some fine, white, granular, crystalline +matter which is probably feldspar. Minute fragments of this rock are easily +fusible. +</p> + +<h3>FOURTHLY.</h3> + +<p> +A compact crystalline rock, banded in straight lines with innumerable layers of +white and grey shades of colour, varying in width from the thirtieth to the +two-hundredth of an inch; these layers seem to be composed chiefly of feldspar, +and they contain numerous perfect crystals of glassy feldspar, which are placed +lengthways; they are also thickly studded with microscopically minute, +amorphous, black specks, which are placed in rows, either standing separately, +or more frequently united, two or three or several together, into black lines, +thinner than a hair. When a small fragment is heated in the blowpipe, the black +specks are easily fused into black brilliant beads, which become +magnetic,—characters that apply to no common mineral except hornblende or +augite. With the black specks there are mingled some others of a red colour, +which are magnetic before being heated, and no doubt are oxide of iron. Round +two little cavities, in a specimen of this variety, I found the black specks +aggregated into minute crystals, appearing like those of augite or hornblende, +but too dull and small to be measured by the goniometer; in the specimen, also, +I could distinguish amidst the crystalline feldspar, grains, which had the +aspect of quartz. By trying with a parallel ruler, I found that the thin grey +layers and the black hair-like lines were absolutely straight and parallel to +each other. It is impossible to trace the gradation from the homogeneous grey +rocks to these striped varieties, or indeed the character of the different +layers in the same specimen, without feeling convinced that the more or less +perfect whiteness of the crystalline feldspathic matter depends on the more or +less perfect aggregation of diffused matter, into the black and red specks of +hornblende and oxide of iron. +</p> + +<h3>FIFTHLY.</h3> + +<p> +A compact heavy rock, not laminated, with an irregular, angular, highly +crystalline, fracture; it abounds with distinct crystals of glassy feldspar, +and the crystalline feldspathic base is mottled with a black mineral, which on +the weathered surface is seen to be aggregated into small crystals, some +perfect, but the greater number imperfect. I showed this specimen to an +experienced geologist, and asked him what it was; he answered, as I think every +one else would have done, that it was a primitive greenstone. The weathered +surface, also, of the banded variety in Figure 4, strikingly resembles a worn +fragment of finely laminated gneiss. +</p> + +<p> +These five varieties, with many intermediate ones, pass and repass into each +other. As the compact varieties are quite subordinate to the others, the whole +may be considered as laminated or striped. The laminae, to sum up their +characteristics, are either quite straight, or slightly tortuous, or +convoluted; they are all parallel to each other, and to the intercalating +strata of obsidian; they are generally of extreme thinness; they consist either +of an apparently homogeneous, compact rock, striped with different shades of +grey and brown colours, or of crystalline feldspathic layers in a more or less +perfect state of purity, and of different thicknesses, with distinct crystals +of glassy feldspar placed lengthways, or of very thin layers chiefly composed +of minute crystals of quartz and augite, or composed of black and red specks of +an augitic mineral and of an oxide of iron, either not crystallised or +imperfectly so. After having fully described the obsidian, I shall return to +the subject of the lamination of rocks of the trachytic series. +</p> + +<p> +The passage of the foregoing beds into the strata of glassy obsidian is +effected in several ways: first, angulo-modular masses of obsidian, both large +and small, abruptly appear disseminated in a slaty, or in an amorphous, +pale-coloured, feldspathic rock, with a somewhat pearly fracture. Secondly, +small irregular nodules of the obsidian, either standing separately, or united +into thin layers, seldom more than the tenth of an inch in thickness, alternate +repeatedly with very thin layers of a feldspathic rock, which is striped with +the finest parallel zones of colour, like an agate, and which sometimes passes +into the nature of pitchstone; the interstices between the nodules of obsidian +are generally filled by soft white matter, resembling pumiceous ashes. Thirdly, +the whole substance of the bounding rock suddenly passes into an +angulo-concretionary mass of obsidian. Such masses (as well as the small +nodules) of obsidian are of a pale green colour, and are generally streaked +with different shades of colour, parallel to the laminae of the surrounding +rock; they likewise generally contain minute white sphaerulites, of which half +is sometimes embedded in a zone of one shade of colour, and half in a zone of +another shade. The obsidian assumes its jet black colour and perfectly +conchoidal fracture, only when in large masses; but even in these, on careful +examination and on holding the specimens in different lights, I could generally +distinguish parallel streaks of different shades of darkness. +</p> + +<p> +(FIGURE 6. OPAQUE BROWN SPHAERULITES, drawn on an enlarged scale. The upper +ones are externally marked with parallel ridges. The internal radiating +structure of the lower ones, is much too plainly represented. +</p> + +<p> +FIGURE 7. A LAYER FORMED BY THE UNION OF MINUTE BROWN SPHAERULITES, +INTERSECTING TWO OTHER SIMILAR LAYERS: the whole represented of nearly the +natural size.) +</p> + +<p> +One of the commonest transitional rocks deserves in several respects a further +description. It is of a very complicated nature, and consists of numerous thin, +slightly tortuous layers of a pale-coloured feldspathic stone, often passing +into an imperfect pitchstone, alternating with layers formed of numberless +little globules of two varieties of obsidian, and of two kinds of sphaerulites, +embedded in a soft or in a hard pearly base. The sphaerulites are either white +and translucent, or dark brown and opaque; the former are quite spherical, of +small size, and distinctly radiated from their centre. The dark brown +sphaerulites are less perfectly round, and vary in diameter from the twentieth +to the thirtieth of an inch; when broken they exhibit towards their centres, +which are whitish, an obscure radiating structure; two of them when united +sometimes have only one central point of radiation; there is occasionally a +trace of or a hollow crevice in their centres. They stand either separately, or +are united two or three or many together into irregular groups, or more +commonly into layers, parallel to the stratification of the mass. This union in +many cases is so perfect, that the two sides of the layer thus formed, are +quite even; and these layers, as they become less brown and opaque, cannot be +distinguished from the alternating layers of the pale-coloured feldspathic +stone. The sphaerulites, when not united, are generally compressed in the plane +of the lamination of the mass; and in this same plane, they are often marked +internally, by zones of different shades of colour, and externally by small +ridges and furrows. In the upper part of Figure 6, the sphaerulites with the +parallel ridges and furrows are represented on an enlarged scale, but they are +not well executed; and in the lower part, their usual manner of grouping is +shown. In another specimen, a thin layer formed of the brown sphaerulites +closely united together, intersects, as represented in Figure 7, a layer of +similar composition; and after running for a short space in a slightly curved +line, again intersects it, and likewise a second layer lying a little way +beneath that first intersected. The small nodules also of obsidian are +sometimes externally marked with ridges and furrows, parallel to the lamination +of the mass, but always less plainly than the sphaerulites. These obsidian +nodules are generally angular, with their edges blunted: they are often +impressed with the form of the adjoining sphaerulites, than which they are +always larger; the separate nodules seldom appear to have drawn each other out +by exerting a mutually attractive force. Had I not found in some cases, a +distinct centre of attraction in these nodules of obsidian, I should have been +led to have considered them as residuary matter, left during the formation of +the pearlstone, in which they are embedded, and of the sphaerulitic globules. +</p> + +<p> +The sphaerulites and the little nodules of obsidian in these rocks so closely +resemble, in general form and structure, concretions in sedimentary deposits, +that one is at once tempted to attribute to them an analogous origin. They +resemble ordinary concretions in the following respects: in their external +form,—in the union of two or three, or of several, into an irregular +mass, or into an even-sided layer,—in the occasional intersection of one +such layer by another, as in the case of chalk-flints,- -in the presence of two +or three kinds of nodules, often close together, in the same basis,—in +their fibrous, radiating structure, with occasional hollows in their +centres,—in the co-existence of a laminary, concretionary, and radiating +structure, as is so well developed in the concretions of magnesian limestone, +described by Professor Sedgwick. (“Geological Transactions” volume +3 part 1 page 37.) Concretions in sedimentary deposits, it is known, are due to +the separation from the surrounding mass of the whole or part of some mineral +substance, and its aggregation round certain points of attraction. Guided by +this fact, I have endeavoured to discover whether obsidian and the sphaerulites +(to which may be added marekanite and pearlstone, both of them occurring in +nodular concretions in the trachytic series) differ in their constituent parts, +from the minerals generally composing trachytic rocks. It appears from three +analyses, that obsidian contains on an average 76 per cent of silica; from one +analysis, that sphaerulites contain 79.12; from two, that marekanite contains +79.25; and from two other analyses, that pearlstone contains 75.62 of silica. +(The foregoing analyses are taken from Beudant “Traite de +Mineralogie” tome 2 page 113; and one analysis of obsidian from Phillips +“Mineralogy.”) Now, the constituent parts of trachyte, as far as +they can be distinguished consist of feldspar, containing 65.21 of silica; or +of albite, containing 69.09; of hornblende, containing 55.27 (These analyses +are taken from Von Kobell “Grundzuge der Mineralogie” 1838.), and +of oxide of iron: so that the foregoing glassy concretionary substances all +contain a larger proportion of silica than that occurring in ordinary +feldspathic or trachytic rocks. D’Aubuisson (“Traite de +Geogn.” tome 2 page 535.), also, has remarked on the large proportion of +silica compared with alumina, in six analyses of obsidian and pearlstone given +in Brongniart’s “Mineralogy.” Hence I conclude, that the +foregoing concretions have been formed by a process of aggregation, strictly +analogous to that which takes place in aqueous deposits, acting chiefly on the +silica, but likewise on some of the other elements of the surrounding mass, and +thus producing the different concretionary varieties. From the well-known +effects of rapid cooling (This is seen in the manufacture of common glass, and +in Gregory Watts’s experiments on molten trap; also on the natural +surfaces of lava- streams, and on the side-walls of dikes.) in giving +glassiness of texture, it is probably necessary that the entire mass, in cases +like that of Ascension, should have cooled at a certain rate; but considering +the repeated and complicated alterations of nodules and thin layers of a glassy +texture with other layers quite stony or crystalline, all within the space of a +few feet or even inches, it is hardly possible that they could have cooled at +different rates, and thus have acquired their different textures. +</p> + +<p> +The natural sphaerulites in these rocks very closely resemble those produced in +glass, when slowly cooled. (I do not know whether it is generally known, that +bodies having exactly the same appearance as sphaerulites, sometimes occur in +agates. Mr. Robert Brown showed me in an agate, formed within a cavity in a +piece of silicified wood, some little specks, which were only just visible to +the naked eye: these specks, when placed by him under a lens of high power, +presented a beautiful appearance: they were perfectly circular, and consisted +of the finest fibres of a brown colour, radiating with great exactness from a +common centre. These little radiating stars are occasionally intersected, and +portions are quite cut off by the fine, ribbon-like zones of colour in the +agate. In the obsidian of Ascension, the halves of a sphaerulite often lie in +different zones of colour, but they are not cut off by them, as in the agate.) +In some fine specimens of partially devitrified glass, in the possession of Mr. +Stokes, the sphaerulites are united into straight layers with even sides, +parallel to each other, and to one of the outer surfaces, exactly as in the +obsidian. These layers sometimes interbranch and form loops; but I did not see +any case of actual intersection. They form the passage from the perfectly +glassy portions, to those nearly homogeneous and stony, with only an obscure +concretionary structure. In the same specimen, also, sphaerulites differing +slightly in colour and in structure, occur embedded close together. Considering +these facts, it is some confirmation of the view above given of the +concretionary origin of the obsidian and natural sphaerulites, to find that M. +Dartigues (“Journal de Physique” tome 59 1804 pages 10, 12.), in +his curious paper on this subject, attributes the production of sphaerulites in +glass, to the different ingredients obeying their own laws of attraction and +becoming aggregated. He is led to believe that this takes place, from the +difficulty in remelting sphaerulitic glass, without the whole be first +thoroughly pounded and mixed together; and likewise from the fact, that the +change takes place most readily in glass composed of many ingredients. In +confirmation of M. Dartigues’ view, I may remark, that M. Fleuriau de +Bellevue (Idem tome 60 1805 page 418.) found that the sphaerulitic portions of +devitrified glass were acted on both by nitric acid and under the blowpipe, in +a different manner from the compact paste in which they were embedded. +</p> + +<p> +COMPARISON OF THE OBSIDIAN BEDS AND ALTERNATING STRATA OF ASCENSION, WITH THOSE +OF OTHER COUNTRIES. +</p> + +<p> +I have been struck with much surprise, how closely the excellent description of +the obsidian rocks of Hungary, given by Beudant (“Voyage en +Hongrie” tome 1 page 330; tome 2 pages 221 and 315; tome 3 pages 369, +371, 377, 381.), and that by Humboldt, of the same formation in Mexico and Peru +(“Essai Geognostique” pages 176, 326, 328.), and likewise the +descriptions given by several authors (P. Scrope “Geological +Transactions” volume 2 second series page 195. Consult also Dolomieu +“Voyage aux Isles Lipari” and D’Aubuisson “Traite de +Geogn.” tome 2 page 534.) of the trachytic regions in the Italian +islands, agree with my observations at Ascension. Many passages might have been +transferred without alteration from the works of the above authors, and would +have been applicable to this island. They all agree in the laminated and +stratified character of the whole series; and Humboldt speaks of some of the +beds of obsidian being ribboned like jasper. (In Mr. Stokes’ fine +collection of obsidians from Mexico, I observe that the sphaerulites are +generally much larger than those of Ascension; they are generally white, +opaque, and are united into distinct layers: there are many singular varieties, +different from any at Ascension. The obsidians are finely zoned, in quite +straight or curved lines, with exceedingly slight differences of tint, of +cellularity, and of more or less perfect degrees of glassiness. Tracing some of +the less perfectly glassy zones, they are seen to become studded with minute +white sphaerulites, which become more and more numerous, until at last they +unite and form a distinct layer: on the other hand, at Ascension, only the +brown sphaerulites unite and form layers; the white ones always being +irregularly disseminated. Some specimens at the Geological Society, said to +belong to an obsidian formation from Mexico, have an earthy fracture, and are +divided in the finest parallel laminae, by specks of a black mineral, like the +augitic or hornblendic specks in the rocks at Ascension.) They all agree in the +nodular or concretionary character of the obsidian, and of the passage of these +nodules into layers. They all refer to the repeated alterations, often in +undulatory planes, of glassy, pearly, stony, and crystalline layers: the +crystalline layers, however, seem to be much more perfectly developed at +Ascension, than in the above-named countries. Humboldt compares some of the +stony beds, when viewed from a distance, to strata of a schistose sandstone. +Sphaerulites are described as occurring abundantly in all cases; and they +everywhere seem to mark the passage, from the perfectly glassy to the stony and +crystalline beds. Beudant’s account (Beudant “Voyage” tome 3 +page 373.) of his “perlite lithoide globulaire” in every, even the +most trifling particular, might have been written for the little brown +sphaerulitic globules of the rocks of Ascension. +</p> + +<p> +From the close similarity in so many respects, between the obsidian formations +of Hungary, Mexico, Peru, and of some of the Italian islands, with that of +Ascension, I can hardly doubt that in all these cases, the obsidian and the +sphaerulites owe their origin to a concretionary aggregation of the silica, and +of some of the other constituent elements, taking place whilst the liquified +mass cooled at a certain required rate. It is, however, well-known, that in +several places, obsidian has flowed in streams like lava; for instance, at +Teneriffe, at the Lipari Islands, and at Iceland. (For Teneriffe see von Buch +“Descript. des Isles Canaries” pages 184 and 190; for the Lipari +Islands see Dolomieu “Voyage” page 34; for Iceland see Mackenzie +“Travels” page 369.) In these cases, the superficial parts are the +most perfectly glassy, the obsidian passing at the depth of a few feet into an +opaque stone. In an analysis by Vauquelin of a specimen of obsidian from Hecla, +which probably flowed as lava, the proportion of silica is nearly the same as +in the nodular or concretionary obsidian from Mexico. It would be interesting +to ascertain, whether the opaque interior portions and the superficial glassy +coating contained the same proportional constituent parts: we know from M. +Dufrenoy (“Memoires pour servir a une Descript. Geolog. de la +France” tome 4 page 371.) that the exterior and interior parts of the +same stream of lava sometimes differ considerably in their composition. Even +should the whole body of the stream of obsidian turn out to be similarly +composed with nodular obsidian, it would only be necessary, in accordance with +the foregoing facts, to suppose that lava in these instances had been erupted +with its ingredients mixed in the same proportion, as in the concretionary +obsidian. +</p> + +<h3>LAMINATION OF VOLCANIC ROCKS OF THE TRACHYTIC SERIES.</h3> + +<p> +We have seen that, in several and widely distant countries, the strata +alternating with beds of obsidian, are highly laminated. The nodules, also, +both large and small, of the obsidian, are zoned with different shades of +colour; and I have seen a specimen from Mexico in Mr. Stokes’ collection, +with its external surface weathered (MacCulloch states “Classification of +Rocks” page 531 that the exposed surfaces of the pitchstone dikes in +Arran are furrowed “with undulating lines, resembling certain varieties +of marbled paper, and which evidently result from some corresponding difference +of laminar structure.”) into ridges and furrows, corresponding with the +zones of different degrees of glassiness: Humboldt (“Personal +Narrative” volume 1 page 222.), moreover, found on the Peak of Teneriffe, +a stream of obsidian divided by very thin, alternating, layers of pumice. Many +other lavas of the feldspathic series are laminated; thus, masses of common +trachyte at Ascension are divided by fine earthy lines, along which the rock +splits, separating thin layers of slightly different shades of colour; the +greater number, also, of the embedded crystals of glassy feldspar are placed +lengthways in the same direction. Mr. P. Scrope (“Geological +Transactions” volume 2 second series page 195.) has described a +remarkable columnar trachyte in the Panza Islands, which seems to have been +injected into an overlying mass of trachytic conglomerate: it is striped with +zones, often of extreme tenuity, of different textures and colours; the harder +and darker zones appearing to contain a larger proportion of silica. In another +part of the island, there are layers of pearlstone and pitchstone, which in +many respects resemble those of Ascension. The zones in the columnar trachyte +are generally contorted; they extend uninterruptedly for a great length in a +vertical direction, and apparently parallel to the walls of the dike-like mass. +Von Buch (“Description des Iles Canaries” page 184.) has described +at Teneriffe, a stream of lava containing innumerable thin, plate-like crystals +of feldspar, which are arranged like white threads, one behind the other, and +which mostly follow the same direction. Dolomieu (“Voyage aux Isles de +Lipari” pages 35 and 85.) also states, that the grey lavas of the modern +cone of Vulcano, which have a vitreous texture, are streaked with parallel +white lines: he further describes a solid pumice-stone which possesses a +fissile structure, like that of certain micaceous schists. Phonolite, which I +may observe is often, if not always, an injected rock, also, often has a +fissile structure; this is generally due to the parallel position of the +embedded crystals of feldspar, but sometimes, as at Fernando Noronha, seems to +be nearly independent of their presence. (In this case, and in that of the +fissile pumice-stone, the structure is very different from that in the +foregoing cases, where the laminae consist of alternate layers of different +composition or texture. In some sedimentary formations, however, which +apparently are homogeneous and fissile, as in glossy clay-slate, there is +reason to believe, according to D’Aubuisson, that the laminae are really +due to excessively thin, alternating, layers of mica.) From these facts we see, +that various rocks of the feldspathic series have either a laminated or fissile +structure, and that it occurs both in masses which have injected into overlying +strata, and in others which have flowed as streams of lava. +</p> + +<p> +The laminae of the beds, alternating with the obsidian at Ascension, dip at a +high angle under the mountain, at the base of which they are situated; and they +do not appear as if they had been inclined by violence. A high inclination is +common to these beds in Mexico, Peru, and in some of the Italian islands (See +Phillips “Mineralogy” for the Italian Islands page 136. For Mexico +and Peru see Humboldt “Essai Geognostique.” Mr. Edwards also +describes the high inclination of the obsidian rocks of the Cerro del Navaja in +Mexico in the “Proc. of the Geolog. Soc.” June 1838.): on the other +hand, in Hungary, the layers are horizontal; the laminae, also, of some of the +lava-streams above referred to, as far as I can understand the descriptions +given of them, appear to be highly inclined or vertical. I doubt whether in any +of these cases, the laminae have been tilted into their present position; and +in some instances, as in that of the trachyte described by Mr. Scrope, it is +almost certain that they have been originally formed with a high inclination. +In many of these cases, there is evidence that the mass of liquified rock has +moved in the direction of the laminae. At Ascension, many of the air-cells have +a drawn out appearance, and are crossed by coarse semi-glassy fibres, in the +direction of the laminae; and some of the layers, separating the sphaerulitic +globules, have a scored appearance, as if produced by the grating of the +globules. I have seen a specimen of zoned obsidian from Mexico, in Mr. +Stokes’ collection, with the surfaces of the best-defined layers streaked +or furrowed with parallel lines; and these lines or streaks precisely resembled +those, produced on the surface of a mass of artificial glass by its having been +poured out of a vessel. Humboldt, also, has described little cavities, which he +compares to the tails of comets, behind sphaerulites in laminated obsidian +rocks from Mexico, and Mr. Scrope has described other cavities behind fragments +embedded in his laminated trachyte, and which he supposes to have been produced +during the movement of the mass. (“Geological Transactions” volume +2 second series page 200 etc. These embedded fragments, in some instances, +consist of the laminated trachyte broken off and “enveloped in those +parts, which still remained liquid.” Beudant, also, frequently refers in +his great work on “Hungary” tome 3 page 386, to trachytic rocks, +irregularly spotted with fragments of the same varieties, which in other parts +form the parallel ribbons. In these cases, we must suppose, that after part of +the molten mass had assumed a laminated structure, a fresh irruption of lava +broke up the mass, and involved fragments, and that subsequently the whole +became relaminated.) From such facts, most authors have attributed the +lamination of these volcanic rocks to their movement whilst liquified. Although +it is easy to perceive, why each separate air-cell, or each fibre in +pumice-stone (Dolomieu “Voyage” page 64.), should be drawn out in +the direction of the moving mass; it is by no means at first obvious why such +air-cells and fibres should be arranged by the movement, in the same planes, in +laminae absolutely straight and parallel to each other, and often of extreme +tenuity; and still less obvious is it, why such layers should come to be of +slightly different composition and of different textures. +</p> + +<p> +In endeavouring to make out the cause of the lamination of these igneous +feldspathic rocks, let us return to the facts so minutely described at +Ascension. We there see, that some of the thinnest layers are chiefly formed by +numerous, exceedingly minute, though perfect, crystals of different minerals; +that other layers are formed by the union of different kinds of concretionary +globules, and that the layers thus formed, often cannot be distinguished from +the ordinary feldspathic and pitchstone layers, composing a large portion of +the entire mass. The fibrous radiating structure of the sphaerulites seems, +judging from many analogous cases, to connect the concretionary and crystalline +forces: the separate crystals, also, of feldspar all lie in the same parallel +planes. (The formation, indeed, of a large crystal of any mineral in a rock of +mixed composition implies an aggregation of the requisite atoms, allied to +concretionary action. The cause of the crystals of feldspar in these rocks of +Ascension, being all placed lengthways, is probably the same with that which +elongates and flattens all the brown sphaerulitic globules (which behave like +feldspar under the blowpipe) in this same direction.) These allied forces, +therefore, have played an important part in the lamination of the mass, but +they cannot be considered the primary force; for the several kinds of nodules, +both the smallest and largest, are internally zoned with excessively fine +shades of colour, parallel to the lamination of the whole; and many of them +are, also, externally marked in the same direction with parallel ridges and +furrows, which have not been produced by weathering. +</p> + +<p> +Some of the finest streaks of colour in the stony layers, alternating with the +obsidian, can be distinctly seen to be due to an incipient crystallisation of +the constituent minerals. The extent to which the minerals have crystallised +can, also, be distinctly seen to be connected with the greater or less size, +and with the number, of the minute, flattened, crenulated air-cavities or +fissures. Numerous facts, as in the case of geodes, and of cavities in +silicified wood, in primary rocks, and in veins, show that crystallisation is +much favoured by space. Hence, I conclude, that, if in a mass of cooling +volcanic rock, any cause produced in parallel planes a number of minute +fissures or zones of less tension (which from the pent-up vapours would often +be expanded into crenulated air-cavities), the crystallisation of the +constituent parts, and probably the formation of concretions, would be +superinduced or much favoured in such planes; and thus, a laminated structure +of the kind we are here considering would be generated. +</p> + +<p> +That some cause does produce parallel zones of less tension in volcanic rocks, +during their consolidation, we must admit in the case of the thin alternate +layers of obsidian and pumice described by Humboldt, and of the small, +flattened, crenulated air-cells in the laminated rocks of Ascension; for on no +other principle can we conceive why the confined vapours should through their +expansion form air-cells or fibres in separate, parallel planes, instead of +irregularly throughout the mass. In Mr. Stokes’ collection, I have seen a +beautiful example of this structure, in a specimen of obsidian from Mexico, +which is shaded and zoned, like the finest agate, with numerous, straight, +parallel layers, more or less opaque and white, or almost perfectly glassy; the +degree of opacity and glassiness depending on the number of microscopically +minute, flattened air-cells; in this case, it is scarcely possible to doubt but +that the mass, to which the fragment belonged, must have been subjected to +some, probably prolonged, action, causing the tension slightly to vary in the +successive planes. +</p> + +<p> +Several causes appear capable of producing zones of different tension, in +masses semi-liquified by heat. In a fragment of devitrified glass, I have +observed layers of sphaerulites which appeared, from the manner in which they +were abruptly bent, to have been produced by the simple contraction of the mass +in the vessel, in which it cooled. In certain dikes on Mount Etna, described by +M. Elie de Beaumont (“Mem. pour servir” etc. tome 4 page 131.), as +bordered by alternating bands of scoriaceous and compact rock, one is led to +suppose that the stretching movement of the surrounding strata, which +originally produced the fissures, continued whilst the injected rock remained +fluid. Guided, however, by Professor Forbes’ (“Edinburgh New Phil. +Journal” 1842 page 350.) clear description of the zoned structure of +glacier-ice, far the most probable explanation of the laminated structure of +these feldspathic rocks appears to be, that they have been stretched whilst +slowly flowing onwards in a pasty condition (I presume that this is nearly the +same explanation which Mr. Scrope had in his mind, when he speaks +(“Geolog. Transact.” volume 2 second series page 228) of the +ribboned structure of his trachytic rocks, having arisen, from “a linear +extension of the mass, while in a state of imperfect liquidity, coupled with a +concretionary process.”), in precisely the same manner as Professor +Forbes believes, that the ice of moving glaciers is stretched and fissured. In +both cases, the zones may be compared to those in the finest agates; in both, +they extend in the direction in which the mass has flowed, and those exposed on +the surface are generally vertical: in the ice, the porous laminae are rendered +distinct by the subsequent congelation of infiltrated water, in the stony +feldspathic lavas, by subsequent crystalline and concretionary action. The +fragment of glassy obsidian in Mr. Stokes’ collection, which is zoned +with minute air-cells must strikingly resemble, judging from Professor +Forbes’ descriptions, a fragment of the zoned ice; and if the rate of +cooling and nature of the mass had been favourable to its crystallisation or to +concretionary action, we should here have had the finest parallel zones of +different composition and texture. In glaciers, the lines of porous ice and of +minute crevices seem to be due to an incipient stretching, caused by the +central parts of the frozen stream moving faster than the sides and bottom, +which are retarded by friction: hence in glaciers of certain forms and towards +the lower end of most glaciers, the zones become horizontal. May we venture to +suppose that in the feldspathic lavas with horizontal laminae, we see an +analogous case? All geologists, who have examined trachytic regions, have come +to the conclusion, that the lavas of this series have possessed an exceedingly +imperfect fluidity; and as it is evident that only matter thus characterised +would be subject to become fissured and to be formed into zones of different +tensions, in the manner here supposed, we probably see the reason why augitic +lavas, which appear generally to have possessed a high degree of fluidity, are +not, like the feldspathic lavas, divided into laminae of different composition +and texture. (Basaltic lavas, and many other rocks, are not unfrequently +divided into thick laminae or plates, of the same composition, which are either +straight or curved; these being crossed by vertical lines of fissure, sometimes +become united into columns. This structure seems related, in its origin, to +that by which many rocks, both igneous and sedimentary, become traversed by +parallel systems of fissures.) Moreover, in the augitic series, there never +appears to be any tendency to concretionary action, which we have seen plays an +important part in the lamination of rocks, of the trachytic series, or at least +in rendering that structure apparent. +</p> + +<p> +Whatever may be thought of the explanation here advanced of the laminated +structure of the rocks of the trachytic series, I venture to call the attention +of geologists to the simple fact, that in a body of rock at Ascension, +undoubtedly of volcanic origin, layers often of extreme tenuity, quite +straight, and parallel to each other, have been produced;—some composed +of distinct crystals of quartz and diopside, mingled with amorphous augitic +specks and granular feldspar,—others entirely composed of these black +augitic specks, with granules of oxide of iron,—and lastly, others formed +of crystalline feldspar, in a more or less perfect state of purity, together +with numerous crystals of feldspar, placed lengthways. At this island, there is +reason to believe, and in some analogous cases, it is certainly known, that the +laminae have originally been formed with their present high inclination. Facts +of this nature are manifestly of importance, with relation to the structural +origin of that grand series of plutonic rocks, which like the volcanic have +undergone the action of heat, and which consist of alternate layers of quartz, +feldspar, mica and other minerals. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap04"></a>CHAPTER IV.<br /> +ST. HELENA.</h2> + +<p> +Lavas of the feldspathic, basaltic, and submarine series. Section of Flagstaff +Hill and of the Barn. Dikes. Turk’s Cap and Prosperous Bays. Basaltic +ring. Central crateriform ridge, with an internal ledge and a parapet. Cones of +phonolite. Superficial beds of calcareous sandstone. Extinct land-shells. Beds +of detritus. Elevation of the land. Denudation. Craters of elevation. +</p> + +<p> +The whole island is of volcanic origin; its circumference, according to +Beatson, is about twenty-eight miles. (Governor Beatson “Account of St. +Helena.”) The central and largest part consists of rocks of a feldspathic +nature, generally decomposed to an extraordinary degree; and when in this +state, presenting a singular assemblage of alternating, red, purple, brown, +yellow, and white, soft, argillaceous beds. From the shortness of our visit, I +did not examine these beds with care; some of them, especially those of the +white, yellow, and brown shades, originally existed as streams of lava, but the +greater number were probably ejected in the form of scoriae and ashes: other +beds of a purple tint, porphyritic with crystal- shaped patches of a white, +soft substance, which are now unctuous, and yield, like wax, a polished streak +to the nail, seem once to have existed as solid claystone-porphyries: the red +argillaceous beds generally have a brecciated structure, and no doubt have been +formed by the decomposition of scoriae. Several extensive streams, however, +belonging to this series, retain their stony character; these are either of a +blackish-green colour, with minute acicular crystals of feldspar, or of a very +pale tint, and almost composed of minute, often scaly, crystals of feldspar, +abounding with microscopical black specks; they are generally compact and +laminated; others, however, of similar composition, are cellular and somewhat +decomposed. None of these rocks contain large crystals of feldspar, or have the +harsh fracture peculiar to trachyte. These feldspathic lavas and tuffs are the +uppermost or those last erupted; innumerable dikes, however, and great masses +of molten rock, have subsequently been injected into them. They converge, as +they rise, towards the central curved ridge, of which one point attains the +elevation of 2,700 feet. This ridge is the highest land in the island; and it +once formed the northern rim of a great crater, whence the lavas of this series +flowed: from its ruined condition, from the southern half having been removed, +and from the violent dislocation which the whole island has undergone, its +structure is rendered very obscure. +</p> + +<h3>BASALTIC SERIES.</h3> + +<p> +The margin of the island is formed by a rude circle of great, black, +stratified, ramparts of basalt, dipping seaward, and worn into cliffs, which +are often nearly perpendicular, and vary in height from a few hundred feet to +two thousand. This circle, or rather horse-shoe shaped ring, is open to the +south, and is breached by several other wide spaces. Its rim or summit +generally projects little above the level of the adjoining inland country; and +the more recent feldspathic lavas, sloping down from the central heights, +generally abut against and overlap its inner margin; on the north-western side +of the island, however, they appear (judging from a distance) to have flowed +over and concealed portions of it. In some parts, where the basaltic ring has +been breached, and the black ramparts stand detached, the feldspathic lavas +have passed between them, and now overhang the sea-coast in lofty cliffs. The +basaltic rocks are of a black colour and thinly stratified; they are generally +highly vesicular, but occasionally compact; some of them contain numerous +crystals of glassy feldspar and octahedrons of titaniferous iron; others abound +with crystals of augite and grains of olivine. The vesicles are frequently +lined with minute crystals (of chabasie?) and even become amygdaloidal with +them. The streams are separated from each other by cindery matter, or by a +bright red, friable, saliferous tuff, which is marked by successive lines like +those of aqueous deposition; and sometimes it has an obscure, concretionary +structure. The rocks of this basaltic series occur nowhere except near the +coast. In most volcanic districts the trachytic lavas are of anterior origin to +the basaltic; but here we see, that a great pile of rock, closely related in +composition to the trachytic family, has been erupted subsequently to the +basaltic strata: the number, however, of dikes, abounding with large crystals +of augite, with which the feldspathic lavas have been injected, shows perhaps +some tendency to a return to the more usual order of superposition. +</p> + +<h3>BASAL SUBMARINE LAVAS.</h3> + +<p> +The lavas of this basal series lie immediately beneath both the basaltic and +feldspathic rocks. According to Mr. Seale, they may be seen at intervals on the +sea-beach round the entire island. (“Geognosy of the Island of St. +Helena.” Mr. Seale has constructed a gigantic model of St. Helena, well +worth visiting, which is now deposited at Addiscombe College, in Surrey.) In +the sections which I examined, their nature varied much; some of the strata +abound with crystals of augite; others are of a brown colour, either laminated +or in a rubbly condition; and many parts are highly amygdaloidal with +calcareous matter. The successive sheets are either closely united together, or +are separated from each other by beds of scoriaceous rock and of laminated +tuff, frequently containing well-rounded fragments. The interstices of these +beds are filled with gypsum and salt; the gypsum also sometimes occurring in +thin layers. From the large quantity of these two substances, from the presence +of rounded pebbles in the tuffs, and from the abundant amygdaloids, I cannot +doubt that these basal volcanic strata flowed beneath the sea. This remark +ought perhaps to be extended to a part of the superincumbent basaltic rocks; +but on this point, I was not able to obtain clear evidence. The strata of the +basal series, whenever I examined them, were intersected by an extraordinary +number of dikes. +</p> + +<h3>FLAGSTAFF HILL AND THE BARN.</h3> + +<p> +(FIGURE 8. FLAGSTAFF HILL AND THE BARN. (Section West (left) to East (right)) +Flagstaff Hill, 2,272 feet high to The Barn, 2,015 feet high. +</p> + +<p> +The double lines represent the basaltic strata; the single, the basal submarine +strata; the dotted, the upper feldspathic strata; the dikes are shaded +transversely.) +</p> + +<p> +I will now describe some of the more remarkable sections, and will commence +with these two hills, which form the principal external feature on the +north-eastern side of the island. The square, angular outline, and black colour +of the Barn, at once show that it belongs to the basaltic series; whilst the +smooth, conical figure, and the varied bright tints of Flagstaff Hill, render +it equally clear, that it is composed of the softened, feldspathic rocks. These +two lofty hills are connected (as is shown in Figure 8) by a sharp ridge, which +is composed of the rubbly lavas of the basal series. The strata of this ridge +dip westward, the inclination becoming less and less towards the Flagstaff; and +the upper feldspathic strata of this hill can be seen, though with some +difficulty, to dip conformably to the W.S.W. Close to the Barn, the strata of +the ridge are nearly vertical, but are much obscured by innumerable dikes; +under this hill, they probably change from being vertical into being inclined +into an opposite direction; for the upper or basaltic strata, which are about +eight hundred or one thousand feet in thickness, are inclined north-eastward, +at an angle between thirty and forty degrees. +</p> + +<p> +This ridge, and likewise the Barn and Flagstaff Hills, are interlaced by dikes, +many of which preserve a remarkable parallelism in a N.N.W. and S.S.E. +direction. The dikes chiefly consist of a rock, porphyritic with large crystals +of augite; others are formed of a fine-grained and brown- coloured trap. Most +of these dikes are coated by a glossy layer, from one to two-tenths of an inch +in thickness, which, unlike true pitchstone, fuses into a black enamel; this +layer is evidently analogous to the glossy superficial coating of many lava +streams. (This circumstance has been observed (Lyell “Principles of +Geology” volume 4 chapter 10 page 9) in the dikes of the Atrio del +Cavallo, but apparently it is not of very common occurrence. Sir G. Mackenzie, +however, states (page 372 “Travels in Iceland”) that all the veins +in Iceland have a “black vitreous coating on their sides.” Captain +Carmichael, speaking of the dikes in Tristan d’Acunha, a volcanic island +in the Southern Atlantic, says (“Linnaean Transactions” volume 12 +page 485) that their sides, “where they come in contact with the rocks, +are invariably in a semi-vitrified state.”) The dikes can often be +followed for great lengths both horizontally and vertically, and they seem to +preserve a nearly uniform thickness (“Geognosy of the Island of St. +Helena” plate 5.): Mr. Seale states, that one near the Barn, in a height +of 1,260 feet, decreases in width only four inches,—from nine feet at the +bottom, to eight feet and eight inches at the top. On the ridge, the dikes +appear to have been guided in their course, to a considerable degree, by the +alternating soft and hard strata: they are often firmly united to the harder +strata, and they preserve their parallelism for such great lengths, that in +very many instances it was impossible to conjecture, which of the beds were +dikes, and which streams of lava. The dikes, though so numerous on this ridge, +are even more numerous in the valleys a little south of it, and to a degree I +never saw equalled anywhere else: in these valleys they extend in less regular +lines, covering the ground with a network, like a spider’s web, and with +some parts of the surface even appearing to consist wholly of dikes, interlaced +by other dikes. +</p> + +<p> +From the complexity produced by the dikes, from the high inclination and +anticlinal dip of the strata of the basal series, which are overlaid, at the +opposite ends of the short ridge, by two great masses of different ages and of +different composition, I am not surprised that this singular section has been +misunderstood. It has even been supposed to form part of a crater; but so far +is this from having been the case, that the summit of Flagstaff Hill once +formed the lower extremity of a sheet of lava and ashes, which were erupted +from the central, crateriform ridge. Judging from the slope of the +contemporaneous streams in an adjoining and undisturbed part of the island, the +strata of the Flagstaff Hill must have been upturned at least twelve hundred +feet, and probably much more, for the great truncated dikes on its summit show +that it has been largely denuded. The summit of this hill now nearly equals in +height the crateriform ridge; and before having been denuded, it was probably +higher than this ridge, from which it is separated by a broad and much lower +tract of country; we here, therefore, see that the lower extremities of a set +of lava-streams have been tilted up to as great a height as, or perhaps greater +height than, the crater, down the flanks of which they originally flowed. I +believe that dislocations on so grand a scale are extremely rare in volcanic +districts. (M. Constant Prevost “Mem. de la Soc. Geolog.” tome 2 +observes that “les produits volcaniques n’ont que localement et +rarement meme derange le sol, a travers lequel ils se sont fait jour.”) +The formation of such numbers of dikes in this part of the island shows that +the surface must here have been stretched to a quite extraordinary degree: this +stretching, on the ridge between Flagstaff and Barn Hills, probably took place +subsequently (though perhaps immediately so) to the strata being tilted; for +had the strata at that time extended horizontally, they would in all +probability have been fissured and injected transversely, instead of in the +planes of their stratification. Although the space between the Barn and +Flagstaff Hill presents a distinct anticlinal line extending north and south, +and though most of the dikes range with much regularity in the same line, +nevertheless, at only a mile due south of the ridge the strata lie undisturbed. +Hence the disturbing force seems to have acted under a point, rather than along +a line. The manner in which it has acted, is probably explained by the +structure of Little Stony-top, a mountain 2,000 feet high, situated a few miles +southward of the Barn; we there see, even from a distance, a dark-coloured, +sharp, wedge of compact columnar rock, with the bright-coloured feldspathic +strata, sloping away on each side from its uncovered apex. This wedge, from +which it derives its name of Stony-top, consists of a body of rock, which has +been injected whilst liquified into the overlying strata; and if we may suppose +that a similar body of rock lies injected, beneath the ridge connecting the +Barn and Flagstaff, the structure there exhibited would be explained. +</p> + +<h3>TURK’S CAP AND PROSPEROUS BAYS.</h3> + +<p> +(FIGURE 9. PROSPEROUS HILL AND THE BARN. (Section S.S.E. (left) to N.N.W. +(right) Prosperous Hill through Hold-fast-Tom and Flagstaff Hill to The Barn. +</p> + +<p> +The double lines represent the basaltic strata; the single, the basal submarine +strata; the dotted, the upper feldspathic strata.) +</p> + +<p> +Prosperous Hill is a great, black, precipitous mountain, situated two miles and +a half south of the Barn, and composed, like it, of basaltic strata. These +rest, in one part, on the brown-coloured, porphyritic beds of the basal series, +and in another part, on a fissured mass of highly scoriaceous and amygdaloidal +rock, which seems to have formed a small point of eruption beneath the sea, +contemporaneously with the basal series. Prosperous Hill, like the Barn, is +traversed by many dikes, of which the greater number range north and south, and +its strata dip, at an angle of about 20 degrees, rather obliquely from the +island towards the sea. The space between Prosperous Hill and the Barn, as +represented in Figure 9, consists of lofty cliffs, composed of the lavas of the +upper or feldspathic series, which rest, though unconformably, on the basal +submarine strata, as we have seen that they do at Flagstaff Hill. Differently, +however, from in that hill, these upper strata are nearly horizontal, gently +rising towards the interior of the island; and they are composed of +greenish-black, or more commonly, pale brown, compact lavas, instead of +softened and highly coloured matter. These brown-coloured, compact lavas, +consist almost entirely of small glimmering scales, or of minute acicular +crystals, of feldspar, placed close by the side of each other, and abounding +with minute black specks, apparently of hornblende. The basaltic strata of +Prosperous Hill project only a little above the level of the gently-sloping, +feldspathic streams, which wind round and abut against their upturned edges. +The inclination of the basaltic strata seems to be too great to have been +caused by their having flowed down a slope, and they must have been tilted into +their present position before the eruption of the feldspathic streams. +</p> + +<h3>BASALTIC RING.</h3> + +<p> +Proceeding round the Island, the lavas of the upper series, southward of +Prosperous Hill, overhang the sea in lofty precipices. Further on, the +headland, called Great Stony-top, is composed, as I believe, of basalt; as is +Long Range Point, on the inland side of which the coloured beds abut. On the +southern side of the island, we see the basaltic strata of the South Barn, +dipping obliquely seaward at a considerable angle; this headland, also, stands +a little above the level of the more modern, feldspathic lavas. Further on, a +large space of coast, on each side of Sandy Bay, has been much denuded, and +there seems to be left only the basal wreck of the great, central crater. The +basaltic strata reappear, with their seaward dip, at the foot of the hill, +called Man-and-Horse; and thence they are continued along the whole +north-western coast to Sugar-Loaf Hill, situated near to the Flagstaff; and +they everywhere have the same seaward inclination, and rest, in some parts at +least, on the lavas of the basal series. We thus see that the circumference of +the island is formed by a much-broken ring, or rather, a horse-shoe, of basalt, +open to the south, and interrupted on the eastern side by many wide breaches. +The breadth of this marginal fringe on the north-western side, where alone it +is at all perfect, appears to vary from a mile to a mile and a half. The +basaltic strata, as well as those of the subjacent basal series, dip, with a +moderate inclination, where they have not been subsequently disturbed, towards +the sea. The more broken state of the basaltic ring round the eastern half, +compared with the western half of the island, is evidently due to the much +greater denuding power of the waves on the eastern or windward side, as is +shown by the greater height of the cliffs on that side, than to leeward. +Whether the margin of basalt was breached, before or after the eruption of the +lavas of the upper series, is doubtful; but as separate portions of the +basaltic ring appear to have been tilted before that event, and from other +reasons, it is more probable, that some at least of the breaches were first +formed. Reconstructing in imagination, as far as is possible, the ring of +basalt, the internal space or hollow, which has since been filled up with the +matter erupted from the great central crater, appears to have been of an oval +figure, eight or nine miles in length by about four miles in breadth, and with +its axis directed in a N.E. and S W. line, coincident with the present longest +axis of the island. +</p> + +<h3>THE CENTRAL CURVED RIDGE.</h3> + +<p> +This ridge consists, as before remarked, of grey feldspathic lavas, and of red, +brecciated, argillaceous tuffs, like the beds of the upper coloured series. The +grey lavas contain numerous, minute, black, easily fusible specks; and but very +few large crystals of feldspar. They are generally much softened; with the +exception of this character, and of being in many parts highly cellular, they +are quite similar to those great sheets of lava which overhang the coast at +Prosperous Bay. Considerable intervals of time appear to have elapsed, judging +from the marks of denudation, between the formation of the successive beds, of +which this ridge is composed. On the steep northern slope, I observed in +several sections a much worn undulating surface of red tuff, covered by grey, +decomposed, feldspathic lavas, with only a thin earthy layer interposed between +them. In an adjoining part, I noticed a trap-dike, four feet wide, cut off and +covered up by the feldspathic lava, as is represented in Figure 9. The ridge +ends on the eastern side in a hook, which is not represented clearly enough in +any map which I have seen; towards the western end, it gradually slopes down +and divides into several subordinate ridges. The best defined portion between +Diana’s Peak and Nest Lodge, which supports the highest pinnacles in the +island varying from 2,000 to 2,700 feet, is rather less than three miles long +in a straight line. Throughout this space the ridge has a uniform appearance +and structure; its curvature resembles that of the coast-line of a great bay, +being made up of many smaller curves, all open to the south. The northern and +outer side is supported by narrow ridges or buttresses, which slope down to the +adjoining country. The inside is much steeper, and is almost precipitous; it is +formed of the basset edges of the strata, which gently decline outwards. Along +some parts of the inner side, a little way beneath the summit, a flat ledge +extends, which imitates in outline the smaller curvatures of the crest. Ledges +of this kind occur not unfrequently within volcanic craters, and their +formation seems to be due to the sinking down of a level sheet of hardened +lava, the edges of which remain (like the ice round a pool, from which the +water has been drained) adhering to the sides. (A most remarkable instance of +this structure is described in Ellis “Polynesian Researches” second +edition where an admirable drawing is given of the successive ledges or +terraces, on the borders of the immense crater at Hawaii, in the Sandwich +Islands.) +</p> + +<p> +(FIGURE 10. DIKE. (Section showing layers 1, 2 and 3 from top to bottom.) +</p> + +<p> +1. Grey feldspathic lava. +</p> + +<p> +2. A layer, one inch in thickness, of a reddish earthy matter. +</p> + +<p> +3. Brecciated, red, argillaceous tuff.) +</p> + +<p> +In some parts, the ridge is surmounted by a wall or parapet, perpendicular on +both sides. Near Diana’s Peak this wall is extremely narrow. At the +Galapagos Archipelago I observed parapets, having a quite similar structure and +appearance, surmounting several of the craters; one, which I more particularly +examined, was composed of glossy, red scoriae firmly cemented together; being +externally perpendicular, and extending round nearly the whole circumference of +the crater, it rendered it almost inaccessible. The Peak of Teneriffe and +Cotopaxi, according to Humboldt, are similarly constructed; he states that +“at their summits a circular wall surrounds the crater, which wall, at a +distance, has the appearance of a small cylinder placed on a truncated cone. +(“Personal Narrative” volume 1 page 171.) On Cotopaxi this peculiar +structure is visible to the naked eye at more than two thousand toises’ +distance; and no person has ever reached its crater. (Humboldt +“Picturesque Atlas” folio plate 10.) On the Peak of Teneriffe, the +parapet is so high, that it would be impossible to reach the caldera, if on the +eastern side there did not exist a breach.” The origin of these circular +parapets is probably due to the heat or vapours from the crater, penetrating +and hardening the sides to a nearly equal depth, and afterwards to the mountain +being slowly acted on by the weather, which would leave the hardened part, +projecting in the form of a cylinder or circular parapet. +</p> + +<p> +From the points of structure in the central ridge, now +enumerated,—namely, from the convergence towards it of the beds of the +upper series,—from the lavas there becoming highly cellular,—from +the flat ledge, extending along its inner and precipitous side, like that +within some still active craters,—from the parapet-like wall on its +summit,—and lastly, from its peculiar curvature, unlike that of any +common line of elevation, I cannot doubt that this curved ridge forms the last +remnant of a great crater. In endeavouring, however, to trace its former +outline, one is soon baffled; its western extremity gradually slopes down, and, +branching into other ridges, extends to the sea-coast; the eastern end is more +curved, but it is only a little better defined. Some appearances lead me to +suppose that the southern wall of the crater joined the present ridge near Nest +Lodge; in this case the crater must have been nearly three miles long, and +about a mile and a half in breadth. Had the denudation of the ridge and the +decomposition of its constituent rocks proceeded a few steps further, and had +this ridge, like several other parts of the island, been broken up by great +dikes and masses of injected matter, we should in vain have endeavoured to +discover its true nature. Even now we have seen that at Flagstaff Hill the +lower extremity and most distant portion of one sheet of the erupted matter has +been upheaved to as great a height as the crater down which it flowed, and +probably even to a greater height. It is interesting thus to trace the steps by +which the structure of a volcanic district becomes obscured, and finally +obliterated: so near to this last stage is St. Helena, that I believe no one +has hitherto suspected that the central ridge or axis of the island is the last +wreck of the crater, whence the most modern volcanic streams were poured forth. +</p> + +<p> +The great hollow space or valley southward of the central curved ridge, across +which the half of the crater must once have extended, is formed of bare, +water-worn hillocks and ridges of red, yellow, and brown rocks, mingled +together in chaos-like confusion, interlaced by dikes, and without any regular +stratification. The chief part consists of red decomposing scoriae, associated +with various kinds of tuff and yellow argillaceous beds, full of broken +crystals, those of augite being particularly large. Here and there masses of +highly cellular and amygdaloidal lavas protrude. From one of the ridges in the +midst of the valley, a conical precipitous hill, called Lot, boldly stands up, +and forms a most singular and conspicuous object. It is composed of phonolite, +divided in one part into great curved laminae, in another, into angular +concretionary balls, and in a third part into outwardly radiating columns. At +its base the strata of lava, tuff, and scoriae, dip away on all sides (Abich in +his “Views of Vesuvius” plate 6 has shown the manner in which beds, +under nearly similar circumstances, are tilted up. The upper beds are more +turned up than the lower; and he accounts for this, by showing that the lava +insinuates itself horizontally between the lower beds.); the uncovered portion +is 197 feet in height (This height is given by Mr. Seale in his Geognosy of the +island. The height of the summit above the level of the sea is said to be 1,444 +feet.), and its horizontal section gives an oval figure. The phonolite is of a +greenish-grey colour, and is full of minute acicular crystals of feldspar; in +most parts it has a conchoidal fracture, and is sonorous, yet it is crenulated +with minute air-cavities. In a S.W. direction from Lot, there are some other +remarkable columnar pinnacles, but of a less regular shape, namely, Lot’s +Wife, and the Asses’ Ears, composed of allied kinds of rock. From their +flattened shape, and their relative position to each other, they are evidently +connected on the same line of fissure. It is, moreover, remarkable that this +same N.E. and S.W. line, joining Lot and Lot’s Wife, if prolonged would +intersect Flagstaff Hill, which, as before stated, is crossed by numerous dikes +running in this direction, and which has a disturbed structure, rendering it +probable that a great body of once fluid rock lies injected beneath it. +</p> + +<p> +In this same great valley there are several other conical masses of injected +rock (one, I observed, was composed of compact greenstone), some of which are +not connected, as far as is apparent, with any line of dike; whilst others are +obviously thus connected. Of these dikes, three or four great lines stretch +across the valley in a N.E. and S.W. direction, parallel to that one connecting +the Asses’ Ears, Lot’s Wife, and probably Lot. The number of these +masses of injected rock is a remarkable feature in the geology of St. Helena. +Besides those just mentioned, and the hypothetical one beneath Flagstaff Hill, +there is Little Stony-top and others, as I have reason to believe, at the +Man-and-Horse, and at High Hill. Most of these masses, if not all of them, have +been injected subsequently to the last volcanic eruptions from the central +crater. The formation of conical bosses of rock on lines of fissure, the walls +of which are in most cases parallel, may probably be attributed to inequalities +in the tension, causing small transverse fissures, and at these points of +intersection the edges of the strata would naturally yield, and be easily +turned upwards. Finally, I may remark, that hills of phonolite everywhere are +apt to assume singular and even grotesque shapes, like that of Lot +(D’Aubuisson in his “Traite de Geognosie” tome 2 page 540 +particularly remarks that this is the case.): the peak at Fernando Noronha +offers an instance; at St. Jago, however, the cones of phonolite, though +tapering, have a regular form. Supposing, as seems probable, that all such +hillocks or obelisks have originally been injected, whilst liquified, into a +mould formed by yielding strata, as certainly has been the case with Lot, how +are we to account for the frequent abruptness and singularity of their +outlines, compared with similarly injected masses of greenstone and basalt? Can +it be due to a less perfect degree of fluidity, which is generally supposed to +be characteristic of the allied trachytic lavas? +</p> + +<h3>SUPERFICIAL DEPOSITS.</h3> + +<p> +Soft calcareous sandstone occurs in extensive, though thin, superficial beds, +both on the northern and southern shores of the island. It consists of very +minute, equal-sized, rounded particles of shells, and other organic bodies, +which partially retain their yellow, brown, and pink colours, and occasionally, +though very rarely, present an obscure trace of their original external forms. +I in vain endeavoured to find a single unrolled fragment of a shell. The colour +of the particles is the most obvious character by which their origin can be +recognised, the tints being affected (and an odour produced) by a moderate +heat, in the same manner as in fresh shells. The particles are cemented +together, and are mingled with some earthy matter: the purest masses, according +to Beatson, contain 70 per cent of carbonate of lime. The beds, varying in +thickness from two or three feet to fifteen feet, coat the surface of the +ground; they generally lie on that side of the valley which is protected from +the wind, and they occur at the height of several hundred feet above the level +of the sea. Their position is the same which sand, if now drifted by the +trade-wind, would occupy; and no doubt they thus originated, which explains the +equal size and minuteness of the particles, and likewise the entire absence of +whole shells, or even of moderately-sized fragments. It is remarkable that at +the present day there are no shelly beaches on any part of the coast, whence +calcareous dust could be drifted and winnowed; we must, therefore, look back to +a former period when before the land was worn into the present great +precipices, a shelving coast, like that of Ascension, was favourable to the +accumulation of shelly detritus. Some of the beds of this limestone are between +six hundred and seven hundred feet above the sea; but part of this height may +possibly be due to an elevation of the land, subsequent to the accumulation of +the calcareous sand. +</p> + +<p> +The percolation of rain-water has consolidated parts of these beds into a solid +rock, and has formed masses of dark brown, stalagmitic limestone. At the +Sugar-Loaf quarry, fragments of rock on the adjoining slopes have been thickly +coated by successive fine layers of calcareous matter. (In the earthy detritus +on several parts of this hill, irregular masses of very impure, crystallised +sulphate of lime occur. As this substance is now being abundantly deposited by +the surf at Ascension, it is possible that these masses may thus have +originated; but if so, it must have been at a period when the land stood at a +much lower level. This earthy selenite is now found at a height of between six +hundred and seven hundred feet.) It is singular, that many of these pebbles +have their entire surfaces coated, without any point of contact having been +left uncovered; hence, these pebbles must have been lifted up by the slow +deposition between them of the successive films of carbonate of lime. Masses of +white, finely oolitic rock are attached to the outside of some of these coated +pebbles. Von Buch has described a compact limestone at Lanzarote, which seems +perfectly to resemble the stalagmitic deposition just mentioned: it coats +pebbles, and in parts is finely oolitic: it forms a far-extended layer, from +one inch to two or three feet in thickness, and it occurs at the height of 800 +feet above the sea, but only on that side of the island exposed to the violent +north-western winds. Von Buch remarks, that it is not found in hollows, but +only on the unbroken and inclined surfaces of the mountain. (“Description +des Isles Canaries” page 293.) He believes, that it has been deposited by +the spray which is borne over the whole island by these violent winds. It +appears, however, to me much more probable that it has been formed, as at St. +Helena, by the percolation of water through finely comminuted shells: for when +sand is blown on a much-exposed coast, it always tends to accumulate on broad, +even surfaces, which offer a uniform resistance to the winds. At the +neighbouring island, moreover, of Feurteventura, there is an earthy limestone, +which, according to Von Buch, is quite similar to specimens which he has seen +from St. Helena, and which he believes to have been formed by the drifting of +shelly detritus. (Idem pages 314 and 374.) +</p> + +<p> +The upper beds of the limestone, at the above-mentioned quarry on the +Sugar-Loaf Hill, are softer, finer-grained and less pure, than the lower beds. +They abound with fragments of land-shells, and with some perfect ones; they +contain, also, the bones of birds, and the large eggs, apparently of +water-fowl. (Colonel Wilkes, in a catalogue presented with some specimens to +the Geological Society, states that as many as ten eggs were found by one +person. Dr. Buckland has remarked (“Geolog. Trans.” volume 5 page +474) on these eggs.) It is probable that these upper beds remained long in an +unconsolidated form, during which time, these terrestrial productions were +embedded. Mr. G.R. Sowerby has kindly examined three species of land-shells, +which I procured from this bed, and has described them in detail. One of them +is a Succinea, identical with a species now living abundantly on the island; +the two others, namely, Cochlogena fossilis and Helix biplicata, are not known +in a recent state: the latter species was also found in another and different +locality, associated with a species of Cochlogena which is undoubtedly extinct. +</p> + +<h3>BEDS OF EXTINCT LAND-SHELLS.</h3> + +<p> +Land-shells, all of which appear to be species now extinct, occur embedded in +earth, in several parts of the island. The greater number have been found at a +considerable height on Flagstaff Hill. On the N.W. side of this hill, a +rain-channel exposes a section of about twenty feet in thickness, of which the +upper part consists of black vegetable mould, evidently washed down from the +heights above, and the lower part of less black earth, abounding with young and +old shells, and with their fragments: part of this earth is slightly +consolidated by calcareous matter, apparently due to the partial decomposition +of some of the shells. Mr. Seale, an intelligent resident, who first called +attention to these shells, gave me a large collection from another locality, +where the shells appear to have been embedded in very black earth. Mr. G.R. +Sowerby has examined these shells, and has described them. There are seven +species, namely, one Cochlogena, two species of the genus Cochlicopa, and four +of Helix; none of these are known in a recent state, or have been found in any +other country. The smaller species were picked out of the inside of the large +shells of the Cochlogena aurisvulpina. This last-mentioned species is in many +respects a very singular one; it was classed, even by Lamarck, in a marine +genus, and having thus been mistaken for a sea-shell, and the smaller +accompanying species having been overlooked, the exact localities where it was +found have been measured, and the elevation of this island thus deduced! It is +very remarkable that all the shells of this species found by me in one spot, +form a distinct variety, as described by Mr. Sowerby, from those procured from +another locality by Mr. Seale. As this Cochlogena is a large and conspicuous +shell, I particularly inquired from several intelligent countrymen whether they +had ever seen it alive; they all assured me that they had not, and they would +not even believe that it was a land animal: Mr. Seale, moreover, who was a +collector of shells all his life at St. Helena, never met with it alive. +Possibly some of the smaller species may turn out to be yet living kinds; but, +on the other hand, the two land- shells which are now living on the island in +great numbers, do not occur embedded, as far as is yet known, with the extinct +species. I have shown in my “Journal” (“Journal of +Researches” page 582.), that the extinction of these land-shells possibly +may not be an ancient event; as a great change took place in the state of the +island about one hundred and twenty years ago, when the old trees died, and +were not replaced by young ones, these being destroyed by the goats and hogs, +which had run wild in numbers, from the year 1502. Mr. Seale states, that on +Flagstaff Hill, where we have seen that the embedded land-shells are especially +numerous, traces are everywhere discoverable, which plainly indicate that it +was once thickly clothed with trees; at present not even a bush grows there. +The thick bed of black vegetable mould which covers the shell-bed, on the +flanks of this hill, was probably washed down from the upper part, as soon as +the trees perished, and the shelter afforded by them was lost. +</p> + +<h3>ELEVATION OF THE LAND.</h3> + +<p> +Seeing that the lavas of the basal series, which are of submarine origin, are +raised above the level of the sea, and at some places to the height of many +hundred feet, I looked out for superficial signs of the elevation of the land. +The bottoms of some of the gorges, which descend to the coast, are filled up to +the depth of about a hundred feet, by rudely divided layers of sand, muddy +clay, and fragmentary masses; in these beds, Mr. Seale has found the bones of +the tropic-bird and of the albatross; the former now rarely, and the latter +never visiting the island. From the difference between these layers, and the +sloping piles of detritus which rest on them, I suspect that they were +deposited, when the gorges stood beneath the sea. Mr. Seale, moreover, has +shown that some of the fissure- like gorges become, with a concave outline, +gradually rather wider at the bottom than at the top; and this peculiar +structure was probably caused by the wearing action of the sea, when it entered +the lower part of these gorges. (A fissure-like gorge, near Stony-top, is said +by Mr. Seale to be 840 feet deep, and only 115 feet in width.) At greater +heights, the evidence of the rise of the land is even less clear: nevertheless, +in a bay-like depression on the table-land behind Prosperous Bay, at the height +of about a thousand feet, there are flat-topped masses of rock, which it is +scarcely conceivable, could have been insulated from the surrounding and +similar strata, by any other agency than the denuding action of a sea- beach. +Much denudation, indeed, has been effected at great elevations, which it would +not be easy to explain by any other means: thus, the flat summit of the Barn, +which is 2,000 feet high, presents, according to Mr. Seale, a perfect network +of truncated dikes; on hills like the Flagstaff, formed of soft rock, we might +suppose that the dikes had been worn down and cut off by meteoric agency, but +we can hardly suppose this possible with the hard, basaltic strata of the Barn. +</p> + +<h3>COAST DENUDATION.</h3> + +<p> +The enormous cliffs, in many parts between one and two thousand feet in height, +with which this prison-like island is surrounded, with the exception of only a +few places, where narrow valleys descend to the coast, is the most striking +feature in its scenery. We have seen that portions of the basaltic ring, two or +three miles in length by one or two miles in breadth, and from one to two +thousand feet in height, have been wholly removed. There are, also, ledges and +banks of rock, rising out of profoundly deep water, and distant from the +present coast between three and four miles, which, according to Mr. Seale, can +be traced to the shore, and are found to be the continuations of certain +well-known great dikes. The swell of the Atlantic Ocean has obviously been the +active power in forming these cliffs; and it is interesting to observe that the +lesser, though still great, height of the cliffs on the leeward and partially +protected side of the island (extending from the Sugar-Loaf Hill to South West +Point), corresponds with the lesser degree of exposure. When reflecting on the +comparatively low coasts of many volcanic islands, which also stand exposed in +the open ocean, and are apparently of considerable antiquity, the mind recoils +from an attempt to grasp the number of centuries of exposure, necessary to have +ground into mud and to have dispersed the enormous cubic mass of hard rock +which has been pared off the circumference of this island. The contrast in the +superficial state of St. Helena, compared with the nearest island, namely, +Ascension, is very striking. At Ascension, the surfaces of the lava-streams are +glossy, as if just poured forth, their boundaries are well defined, and they +can often be traced to perfect craters, whence they were erupted; in the course +of many long walks, I did not observe a single dike; and the coast round nearly +the entire circumference is low, and has been eaten back (though too much +stress must not be placed on this fact, as the island may have been subsiding) +into a little wall only from ten to thirty feet high. Yet during the 340 years, +since Ascension has been known, not even the feeblest signs of volcanic action +have been recorded. (In the “Nautical Magazine” for 1835 page 642, +and for 1838 page 361, and in the “Comptes Rendus” April 1838, +accounts are given of a series of volcanic +phenomena—earthquakes—troubled water—floating scoriae and +columns of smoke—which have been observed at intervals since the middle +of the last century, in a space of open sea between longitudes 20 degrees and +22 degrees west, about half a degree south of the equator. These facts seem to +show, that an island or an archipelago is in process of formation in the middle +of the Atlantic: a line joining St. Helena and Ascension, prolonged, intersects +this slowly nascent focus of volcanic action.) On the other hand, at St. +Helena, the course of no one stream of lava can be traced, either by the state +of its boundaries or of its superficies; the mere wreck of one great crater is +left; not the valleys only, but the surfaces of some of the highest hills, are +interlaced by worn-down dikes, and, in many places, the denuded summits of +great cones of injected rock stand exposed and naked; lastly, as we have seen, +the entire circuit of the island has been deeply worn back into the grandest +precipices. +</p> + +<h3>CRATERS OF ELEVATION.</h3> + +<p> +There is much resemblance in structure and in geological history between St. +Helena, St. Jago, and Mauritius. All three islands are bounded (at least in the +parts which I was able to examine) by a ring of basaltic mountains, now much +broken, but evidently once continuous. These mountains have, or apparently once +had, their escarpments steep towards the interior of the island, and their +strata dip outwards. I was able to ascertain, only in a few cases, the +inclination of the beds; nor was this easy, for the stratification was +generally obscure, except when viewed from a distance. I feel, however, little +doubt that, according to the researches of M. Elie de Beaumont, their average +inclination is greater than that which they could have acquired, considering +their thickness and compactness, by flowing down a sloping surface. At St. +Helena, and at St. Jago, the basaltic strata rest on older and probably +submarine beds of different composition. At all three islands, deluges of more +recent lavas have flowed from the centre of the island, towards and between the +basaltic mountains; and at St. Helena the central platform has been filled up +by them. All three islands have been raised in mass. At Mauritius the sea, +within a late geological period, must have reached to the foot of the basaltic +mountains, as it now does at St. Helena; and at St. Jago it is cutting back the +intermediate plain towards them. In these three islands, but especially at St. +Jago and at Mauritius, when, standing on the summit of one of the old basaltic +mountains, one looks in vain towards the centre of the island,—the point +towards which the strata beneath one’s feet, and of the mountains on each +side, rudely converge,—for a source whence these strata could have been +erupted; but one sees only a vast hollow platform stretched beneath, or piles +of matter of more recent origin. +</p> + +<p> +These basaltic mountains come, I presume, into the class of Craters of +elevation: it is immaterial whether the rings were ever completely formed, for +the portions which now exist have so uniform a structure, that, if they do not +form fragments of true craters, they cannot be classed with ordinary lines of +elevation. With respect to their origin, after having read the works of Mr. +Lyell (“Principles of Geology” fifth edition volume 2 page 171.), +and of MM. C. Prevost and Virlet, I cannot believe that the great central +hollows have been formed by a simple dome-shaped elevation, and the consequent +arching of the strata. On the other hand, I have very great difficulty in +admitting that these basaltic mountains are merely the basal fragments of great +volcanoes, of which the summits have either been blown off, or more probably +swallowed up by subsidence. These rings are, in some instances, so immense, as +at St. Jago and at Mauritius, and their occurrence is so frequent, that I can +hardly persuade myself to adopt this explanation. Moreover, I suspect that the +following circumstances, from their frequent concurrence, are someway connected +together,—a connection not implied in either of the above views: namely, +first, the broken state of the ring; showing that the now detached portions +have been exposed to great denudation, and in some cases, perhaps, rendering it +probable that the ring never was entire; secondly, the great amount of matter +erupted from the central area after or during the formation of the ring; and +thirdly, the elevation of the district in mass. As far as relates to the +inclination of the strata being greater than that which the basal fragments of +ordinary volcanoes would naturally possess, I can readily believe that this +inclination might have been slowly acquired by that amount of elevation, of +which, according to M. Elie de Beaumont, the numerous upfilled fissures or +dikes are the evidence and the measure,—a view equally novel and +important, which we owe to the researches of that geologist on Mount Etna. +</p> + +<p> +A conjecture, including the above circumstances, occurred to me, when,— +with my mind fully convinced, from the phenomena of 1835 in South America, that +the forces which eject matter from volcanic orifices and raise continents in +mass are identical,—I viewed that part of the coast of St. Jago, where +the horizontally upraised, calcareous stratum dips into the sea, directly +beneath a cone of subsequently erupted lava. (I have given a detailed account +of these phenomena, in a paper read before the Geological Society in March +1838. At the instant of time, when an immense area was convulsed and a large +tract elevated, the districts immediately surrounding several of the great +vents in the Cordillera remained quiescent; the subterranean forces being +apparently relieved by the eruptions, which then recommenced with great +violence. An event of somewhat the same kind, but on an infinitely smaller +scale, appears to have taken place, according to Abich (“Views of +Vesuvius” plates 1 and 9), within the great crater of Vesuvius, where a +platform on one side of a fissure was raised in mass twenty feet, whilst on the +other side, a train of small volcanoes burst forth in eruption.) The conjecture +is that, during the slow elevation of a volcanic district or island, in the +centre of which one or more orifices continue open, and thus relieve the +subterranean forces, the borders are elevated more than the central area; and +that the portions thus upraised do not slope gently into the central, less +elevated area, as does the calcareous stratum under the cone at St. Jago, and +as does a large part of the circumference of Iceland, but that they are +separated from it by curved faults. (It appears, from information communicated +to me in the most obliging manner by M. E. Robert, that the circumferential +parts of Iceland, which are composed of ancient basaltic strata alternating +with tuff, dip inland, thus forming a gigantic saucer. M. Robert found that +this was the case, with a few and quite local exceptions, for a space of coast +several hundred miles in length. I find this statement corroborated, as far as +regards one place, by Mackenzie in his “Travels” page 377, and in +another place by some MS. notes kindly lent me by Dr. Holland. The coast is +deeply indented by creeks, at the head of which the land is generally low. M. +Robert informs me, that the inwardly dipping strata appear to extend as far as +this line, and that their inclination usually corresponds with the slope of the +surface, from the high coast-mountains to the low land at the head of these +creeks. In the section described by Sir G. Mackenzie, the dip is 120. The +interior parts of the island chiefly consist, as far as is known, of recently +erupted matter. The great size, however, of Iceland, equalling the bulkiest +part of England, ought perhaps to exclude it from the class of islands we have +been considering; but I cannot avoid suspecting that if the coast-mountains, +instead of gently sloping into the less elevated central area, had been +separated from it by irregularly curved faults, the strata would have been +tilted seaward, and a “Crater of elevation,” like that of St. Jago +or that of Mauritius, but of much vaster dimensions, would have been formed. I +will only further remark, that the frequent occurrence of extensive lakes at +the foot of large volcanoes, and the frequent association of volcanic and +fresh-water strata, seem to indicate that the areas around volcanoes are apt to +be depressed beneath the level of the adjoining country, either from having +been less elevated, or from the effects of subsidence.) We might expect, from +what we see along ordinary faults, that the strata on the upraised side, +already dipping outwards from their original formation as lava-streams, would +be tilted from the line of fault, and thus have their inclination increased. +According to this hypothesis, which I am tempted to extend only to some few +cases, it is not probable that the ring would ever be formed quite perfect; and +from the elevation being slow, the upraised portions would generally be exposed +to much denudation, and hence the ring become broken; we might also expect to +find occasional inequalities in the dip of the upraised masses, as is the case +at St. Jago. By this hypothesis the elevation of the districts in mass, and the +flowing of deluges of lava from the central platforms, are likewise connected +together. On this view the marginal basaltic mountains of the three foregoing +islands might still be considered as forming “Craters of +elevation;” the kind of elevation implied having been slow, and the +central hollow or platform having been formed, not by the arching of the +surface, but simply by that part having been upraised to a less height. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap05"></a>CHAPTER V.<br /> +GALAPAGOS ARCHIPELAGO.</h2> + +<p> +Chatham Island. Craters composed of a peculiar kind of tuff. Small basaltic +craters, with hollows at their bases. Albemarle Island; fluid lavas, their +composition. Craters of tuff; inclination of their exterior diverging strata, +and structure of their interior converging strata. James Island, segment of a +small basaltic crater; fluidity and composition of its lava-streams, and of its +ejected fragments. Concluding remarks on the craters of tuff, and on the +breached condition of their southern sides. Mineralogical composition of the +rocks of the archipelago. Elevation of the land. Direction of the fissures of +eruption. +</p> + +<h3>(FIGURE 11. MAP 3. GALAPAGOS ARCHIPELAGO.</h3> + +<p> +Showing Wenman, Abingdon, Bindloes, Tower, Narborough, Albemarle, James, +Indefatigable, Barrington, Chatham, Charles and Hood’s Islands.) +</p> + +<p> +This archipelago is situated under the equator, at a distance of between five +and six hundred miles from the west coast of South America. It consists of five +principal islands, and of several small ones, which together are equal in area, +but not in extent of land, to Sicily, conjointly with the Ionian Islands. (I +exclude from this measurement, the small volcanic islands of Culpepper and +Wenman, lying seventy miles northward of the group. Craters were visible on all +the islands of the group, except on Towers Island, which is one of the lowest; +this island is, however, formed of volcanic rocks.) They are all volcanic: on +two, craters have been seen in eruption, and on several of the other islands, +streams of lava have a recent appearance. The larger islands are chiefly +composed of solid rock, and they rise with a tame outline to a height of +between one and four thousand feet. They are sometimes, but not generally, +surmounted by one principal orifice. The craters vary in size from mere +spiracles to huge caldrons several miles in circumference; they are +extraordinarily numerous, so that I should think, if enumerated, they would be +found to exceed two thousand; they are formed either of scoriae and lava, or of +a brown-coloured tuff; and these latter craters are in several respects +remarkable. The whole group was surveyed by the officers of the +“Beagle.” I visited myself four of the principal islands, and +received specimens from all the others. Under the head of the different islands +I will describe only that which appears to me deserving of attention. +</p> + +<h3>CHATHAM ISLAND. CRATERS COMPOSED OF A SINGULAR KIND OF TUFF.</h3> + +<p> +Towards the eastern end of this island there occur two craters composed of two +kinds of tuff; one kind being friable, like slightly consolidated ashes; and +the other compact, and of a different nature from anything which I have met +with described. This latter substance, where it is best characterised, is of a +yellowish-brown colour, translucent, and with a lustre somewhat resembling +resin; it is brittle, with an angular, rough, and very irregular fracture, +sometimes, however, being slightly granular, and even obscurely crystalline: it +can readily be scratched with a knife, yet some points are hard enough just to +mark common glass; it fuses with ease into a blackish-green glass. The mass +contains numerous broken crystals of olivine and augite, and small particles of +black and brown scoriae; it is often traversed by thin seams of calcareous +matter. It generally affects a nodular or concretionary structure. In a hand +specimen, this substance would certainly be mistaken for a pale and peculiar +variety of pitchstone; but when seen in mass its stratification, and the +numerous layers of fragments of basalt, both angular and rounded, at once +render its subaqueous origin evident. An examination of a series of specimens +shows that this resin-like substance results from a chemical change on small +particles of pale and dark-coloured scoriaceous rocks; and this change could be +distinctly traced in different stages round the edges of even the same +particle. The position near the coast of all the craters composed of this kind +of tuff or peperino, and their breached condition, renders it probable that +they were all formed when standing immersed in the sea; considering this +circumstance, together with the remarkable absence of large beds of ashes in +the whole archipelago, I think it highly probable that much the greater part of +the tuff has originated from the trituration of fragments of the grey, basaltic +lavas in the mouths of craters standing in the sea. It may be asked whether the +heated water within these craters has produced this singular change in the +small scoriaceous particles and given to them their translucent, resin-like +fracture. Or has the associated lime played any part in this change? I ask +these questions from having found at St. Jago, in the Cape de Verde Islands, +that where a great stream of molten lava has flowed over a calcareous bottom +into the sea, the outermost film, which in other parts resembles pitchstone, is +changed, apparently by its contact with the carbonate of lime, into a +resin-like substance, precisely like the best characterised specimens of the +tuff from this archipelago. (The concretions containing lime, which I have +described at Ascension, as formed in a bed of ashes, present some degree of +resemblance to this substance, but they have not a resinous fracture. At St. +Helena, also, I found veins of a somewhat similar, compact, but non- resinous +substance, occurring in a bed of pumiceous ashes, apparently free from +calcareous matter: in neither of these cases could heat have acted.) +</p> + +<p> +To return to the two craters: one of them stands at the distance of a league +from the coast, the intervening tract consisting of a calcareous tuff, +apparently of submarine origin. This crater consists of a circle of hills some +of which stand quite detached, but all have a very regular, qua- qua versal +dip, at an inclination of between thirty and forty degrees. The lower beds, to +the thickness of several hundred feet, consist of the resin- like stone, with +embedded fragments of lava. The upper beds, which are between thirty and forty +feet in thickness, are composed of a thinly stratified, fine-grained, harsh, +friable, brown-coloured tuff, or peperino. (Those geologists who restrict the +term of “tuff” to ashes of a white colour, resulting from the +attrition of feldspathic lavas, would call these brown-coloured strata +“peperino.”) A central mass without any stratification, which must +formerly have occupied the hollow of the crater, but is now attached only to a +few of the circumferential hills, consists of a tuff, intermediate in character +between that with a resin-like, and that with an earthy fracture. This mass +contains white calcareous matter in small patches. The second crater (520 feet +in height) must have existed until the eruption of a recent, great stream of +lava, as a separate islet; a fine section, worn by the sea, shows a grand +funnel-shaped mass of basalt, surrounded by steep, sloping flanks of tuff, +having in parts an earthy, and in others a semi-resinous fracture. The tuff is +traversed by several broad, vertical dikes, with smooth and parallel sides, +which I did not doubt were formed of basalt, until I actually broke off +fragments. These dikes, however, consist of tuff like that of the surrounding +strata, but more compact, and with a smoother fracture; hence we must conclude, +that fissures were formed and filled up with the finer mud or tuff from the +crater, before its interior was occupied, as it now is, by a solidified pool of +basalt. Other fissures have been subsequently formed, parallel to these +singular dikes, and are merely filled with loose rubbish. The change from +ordinary scoriaceous particles to the substance with a semi-resinous fracture, +could be clearly followed in portions of the compact tuff of these dikes. +</p> + +<h3>(FIGURE 12. THE KICKER ROCK, 400 FEET HIGH.)</h3> + +<p> +At the distance of a few miles from these two craters, stands the Kicker Rock, +or islet, remarkable from its singular form. It is unstratified, and is +composed of compact tuff, in parts having the resin-like fracture. It is +probable that this amorphous mass, like that similar mass in the case first +described, once filled up the central hollow of a crater, and that its flanks, +or sloping walls, have since been worn quite away by the sea, in which it +stands exposed. +</p> + +<h3>SMALL BASALTIC CRATERS.</h3> + +<p> +A bare, undulating tract, at the eastern end of Chatham Island, is remarkable +from the number, proximity, and form of the small basaltic craters with which +it is studded. They consist, either of a mere conical pile, or, but less +commonly, of a circle, of black and red, glossy scoriae, partially cemented +together. They vary in diameter from thirty to one hundred and fifty yards, and +rise from about fifty to one hundred feet above the level of the surrounding +plain. From one small eminence, I counted sixty of these craters, all of which +were within a third of a mile from each other, and many were much closer. I +measured the distance between two very small craters, and found that it was +only thirty yards from the summit-rim of one to the rim of the other. Small +streams of black, basaltic lava, containing olivine and much glassy feldspar, +have flowed from many, but not from all of these craters. The surfaces of the +more recent streams were exceedingly rugged, and were crossed by great +fissures; the older streams were only a little less rugged; and they were all +blended and mingled together in complete confusion. The different growth, +however, of the trees on the streams, often plainly marked their different +ages. Had it not been for this latter character, the streams could in few cases +have been distinguished; and, consequently, this wide undulatory tract might +have (as probably many tracts have) been erroneously considered as formed by +one great deluge of lava, instead of by a multitude of small streams, erupted +from many small orifices. +</p> + +<p> +In several parts of this tract, and especially at the base of the small +craters, there are circular pits, with perpendicular sides, from twenty to +forty feet deep. At the foot of one small crater, there were three of these +pits. They have probably been formed, by the falling in of the roofs of small +caverns. (M. Elie de Beaumont has described (“Mem. pour servir” +etc. tome 4 page 113) many “petits cirques d’eboulement” on +Etna, of some of which the origin is historically known.) In other parts, there +are mammiform hillocks, which resemble great bubbles of lava, with their +summits fissured by irregular cracks, which appeared, upon entering them, to be +very deep; lava has not flowed from these hillocks. There are, also, other very +regular, mammiform hillocks, composed of stratified lava, and surmounted by +circular, steep-sided hollows, which, I suppose have been formed by a body of +gas, first, arching the strata into one of the bubble- like hillocks, and then, +blowing off its summit. These several kinds of hillocks and pits, as well as +the numerous, small, scoriaceous craters, all show that this tract has been +penetrated, almost like a sieve, by the passage of heated vapours. The more +regular hillocks could only have been heaved up, whilst the lava was in a +softened state. (Sir G. Mackenzie “Travels in Iceland” pages 389 to +392, has described a plain of lava at the foot of Hecla, everywhere heaved up +into great bubbles or blisters. Sir George states that this cavernous lava +composes the uppermost stratum; and the same fact is affirmed by Von Buch +“Descript. des Isles Canaries” page 159, with respect to the +basaltic stream near Rialejo, in Teneriffe. It appears singular that it should +be the upper streams that are chiefly cavernous, for one sees no reason why the +upper and lower should not have been equally affected at different +times;—have the inferior streams flowed beneath the pressure of the sea, +and thus been flattened, after the passage through them, of bodies of gas?) +</p> + +<h3>ALBEMARLE ISLAND.</h3> + +<p> +This island consists of five, great, flat-topped craters, which, together with +the one on the adjoining island of Narborough, singularly resemble each other, +in form and height. The southern one is 4,700 feet high, two others are 3,720 +feet, a third only 50 feet higher, and the remaining ones apparently of nearly +the same height. Three of these are situated on one line, and their craters +appear elongated in nearly the same direction. The northern crater, which is +not the largest, was found by the triangulation to measure, externally, no less +than three miles and one-eighth of a mile in diameter. Over the lips of these +great, broad caldrons, and from little orifices near their summits, deluges of +black lava have flowed down their naked sides. +</p> + +<h3>FLUIDITY OF DIFFERENT LAVAS.</h3> + +<p> +Near Tagus or Banks’ Cove, I examined one of these great streams of lava, +which is remarkable from the evidence of its former high degree of fluidity, +especially when its composition is considered. Near the sea-coast this stream +is several miles in width. It consists of a black, compact base, easily fusible +into a black bead, with angular and not very numerous air-cells, and thickly +studded with large, fractured crystals of glassy albite, varying from the tenth +of an inch to half an inch in diameter. (In the Cordillera of Chile, I have +seen lava very closely resembling this variety at the Galapagos Archipelago. It +contained, however, besides the albite, well-formed crystals of augite, and the +base (perhaps in consequence of the aggregation of the augitic particles) was a +shade lighter in colour. I may here remark, that in all these cases, I call the +feldspathic crystals, “albite,” from their cleavage-planes (as +measured by the reflecting goniometer) corresponding with those of that +mineral. As, however, other species of this genus have lately been discovered +to cleave in nearly the same planes with albite, this determination must be +considered as only provisional. I examined the crystals in the lavas of many +different parts of the Galapagos group, and I found that none of them, with the +exception of some crystals from one part of James Island, cleaved in the +direction of orthite or potash-feldspar.) This lava, although at first sight +appearing eminently porphyritic, cannot properly be considered so, for the +crystals have evidently been enveloped, rounded, and penetrated by the lava, +like fragments of foreign rock in a trap-dike. This was very clear in some +specimens of a similar lava, from Abingdon Island, in which the only difference +was, that the vesicles were spherical and more numerous. The albite in these +lavas is in a similar condition with the leucite of Vesuvius, and with the +olivine, described by Von Buch, as projecting in great balls from the basalt of +Lanzarote. (“Description des Isles Canaries” page 295.) Besides the +albite, this lava contains scattered grains of a green mineral, with no +distinct cleavage, and closely resembling olivine (Humboldt mentions that he +mistook a green augitic mineral, occurring in the volcanic rocks of the +Cordillera of Quito, for olivine.); but as it fuses easily into a green glass, +it belongs probably to the augitic family: at James Island, however, a similar +lava contained true olivine. I obtained specimens from the actual surface, and +from a depth of four feet, but they differed in no respect. The high degree of +fluidity of this lava-stream was at once evident, from its smooth and gently +sloping surface, from the manner in which the main stream was divided by small +inequalities into little rills, and especially from the manner in which its +edges, far below its source, and where it must have been in some degree cooled, +thinned out to almost nothing; the actual margin consisting of loose fragments, +few of which were larger than a man’s head. The contrast between this +margin, and the steep walls, above twenty feet high, bounding many of the +basaltic streams at Ascension, is very remarkable. It has generally been +supposed that lavas abounding with large crystals, and including angular +vesicles, have possessed little fluidity; but we see that the case has been +very different at Albemarle Island. (The irregular and angular form of the +vesicles is probably caused by the unequal yielding of a mass composed, in +almost equal proportion, of solid crystals and of a viscid base. It certainly +seems a general circumstance, as might have been expected, that in lava, which +has possessed a high degree of fluidity, AS WELL AS AN EVEN-SIZED GRAIN, the +vesicles are internally smooth and spherical.) The degree of fluidity in +different lavas, does not seem to correspond with any APPARENT corresponding +amount of difference in their composition: at Chatham Island, some streams, +containing much glassy albite and some olivine, are so rugged, that they may be +compared to a sea frozen during a storm; whilst the great stream at Albemarle +Island is almost as smooth as a lake when ruffled by a breeze. At James Island, +black basaltic lava, abounding with small grains of olivine, presents an +intermediate degree of roughness; its surface being glossy, and the detached +fragments resembling, in a very singular manner, folds of drapery, cables, and +pieces of the bark of trees. (A specimen of basaltic lava, with a few small +broken crystals of albite, given me by one of the officers, is perhaps worthy +of description. It consists of cylindrical ramifications, some of which are +only the twentieth of an inch in diameter, and are drawn out into the sharpest +points. The mass has not been formed like a stalactite, for the points +terminate both upwards and downwards. Globules, only the fortieth of an inch in +diameter, have dropped from some of the points, and adhere to the adjoining +branches. The lava is vesicular, but the vesicles never reach the surface of +the branches, which are smooth and glossy. As it is generally supposed that +vesicles are always elongated in the direction of the movement of the fluid +mass, I may observe, that in these cylindrical branches, which vary from a +quarter to only the twentieth of an inch in diameter, every air-cell is +spherical.) +</p> + +<h3>CRATERS OF TUFF.</h3> + +<p> +About a mile southward of Banks’ Cove, there is a fine elliptic crater, +about five hundred feet in depth, and three-quarters of a mile in diameter. Its +bottom is occupied by a lake of brine, out of which some little crateriform +hills of tuff rise. The lower beds are formed of compact tuff, appearing like a +subaqueous deposit; whilst the upper beds, round the entire circumference, +consist of a harsh, friable tuff, of little specific gravity, but often +containing fragments of rock in layers. This upper tuff contains numerous +pisolitic balls, about the size of small bullets, which differ from the +surrounding matter, only in being slightly harder and finer grained. The beds +dip away very regularly on all sides, at angles varying, as I found by +measurement, from twenty-five to thirty degrees. The external surface of the +crater slopes at a nearly similar inclination, and is formed by slightly convex +ribs, like those on the shell of a pecten or scallop, which become broader as +they extend from the mouth of the crater to its base. These ribs are generally +from eight to twenty feet in breadth, but sometimes they are as much as forty +feet broad; and they resemble old, plastered, much flattened vaults, with the +plaster scaling off in plates: they are separated from each other by gullies, +deepened by alluvial action. At their upper and narrow ends, near the mouth of +the crater, these ribs often consist of real hollow passages, like, but rather +smaller than, those often formed by the cooling of the crust of a lava-stream, +whilst the inner parts have flowed onward;—of which structure I saw many +examples at Chatham Island. There can be no doubt but that these hollow ribs or +vaults have been formed in a similar manner, namely, by the setting or +hardening of a superficial crust on streams of mud, which have flowed down from +the upper part of the crater. In another part of this same crater, I saw open +concave gutters between one and two feet wide, which appear to have been formed +by the hardening of the lower surface of a mud stream, instead of, as in the +former case, of the upper surface. From these facts I think it is certain that +the tuff must have flowed as mud. (This conclusion is of some interest, because +M. Dufrenoy “Mem. pour servir” tome 4 page 274, has argued from +strata of tuff, apparently of similar composition with that here described, +being inclined at angles between 18 degrees and 20 degrees, that Monte Nuevo +and some other craters of Southern Italy have been formed by upheaval. From the +facts given above, of the vaulted character of the separate rills, and from the +tuff not extending in horizontal sheets round these crateriform hills, no one +will suppose that the strata have here been produced by elevation; and yet we +see that their inclination is above 20 degrees, and often as much as 30 +degrees. The consolidated strata also, of the internal talus, as will be +immediately seen, dips at an angle of above 30 degrees.) This mud may have been +formed either within the crater, or from ashes deposited on its upper parts, +and afterwards washed down by torrents of rain. The former method, in most of +the cases, appears the more probable one; at James Island, however, some beds +of the friable kind of tuff extend so continuously over an uneven surface, that +probably they were formed by the falling of showers of ashes. +</p> + +<p> +Within this same crater, strata of coarse tuff, chiefly composed of fragments +of lava, abut, like a consolidated talus, against the inside walls. They rise +to a height of between one hundred and one hundred and fifty feet above the +surface of the internal brine-lake; they dip inwards, and are inclined at an +angle varying from thirty to thirty-six degrees. They appear to have been +formed beneath water, probably at a period when the sea occupied the hollow of +the crater. I was surprised to observe that beds having this great inclination +did not, as far as they could be followed, thicken towards their lower +extremities. +</p> + +<h3>BANKS’ COVE.</h3> + +<p> +(FIGURE 13. A SECTIONAL SKETCH OF THE HEADLANDS FORMING BANKS’ COVE, +showing the diverging crateriform strata, and the converging stratified talus. +The highest point of these hills is 817 feet above the sea.) +</p> + +<p> +This harbour occupies part of the interior of a shattered crater of tuff larger +than that last described. All the tuff is compact, and includes numerous +fragments of lava; it appears like a subaqueous deposit. The most remarkable +feature in this crater is the great development of strata converging inwards, +as in the last case, at a considerable inclination, and often deposited in +irregular curved layers. These interior converging beds, as well as the proper, +diverging crateriform strata, are represented in Figure 13, a rude, sectional +sketch of the headlands, forming this Cove. The internal and external strata +differ little in composition, and the former have evidently resulted from the +wear and tear, and redeposition of the matter forming the external crateriform +strata. From the great development of these inner beds, a person walking round +the rim of this crater might fancy himself on a circular anticlinal ridge of +stratified sandstone and conglomerate. The sea is wearing away the inner and +outer strata, and especially the latter; so that the inwardly converging strata +will, perhaps, in some future age, be left standing alone—a case which +might at first perplex a geologist. (I believe that this case actually occurs +in the Azores, where Dr. Webster “Description” page 185, has +described a basin-formed, little island, composed of STRATA OF TUFF, dipping +inwards and bounded externally by steep sea-worn cliffs. Dr. Daubeny supposes +“Volcanoes” page 266, that this cavity must have been formed by a +circular subsidence. It appears to me far more probable, that we here have +strata which were originally deposited within the hollow of a crater, of which +the exterior walls have since been removed by the sea.) +</p> + +<h3>JAMES ISLAND.</h3> + +<p> +Two craters of tuff on this island are the only remaining ones which require +any notice. One of them lies a mile and a half inland from Puerto Grande: it is +circular, about the third of a mile in diameter, and 400 feet in depth. It +differs from all the other tuff-craters which I examined, in having the lower +part of its cavity, to the height of between one hundred and one hundred and +fifty feet, formed by a precipitous wall of basalt, giving to the crater the +appearance of having burst through a solid sheet of rock. The upper part of +this crater consists of strata of the altered tuff, with a semi-resinous +fracture. Its bottom is occupied by a shallow lake of brine, covering layers of +salt, which rest on deep black mud. The other crater lies at the distance of a +few miles, and is only remarkable from its size and perfect condition. Its +summit is 1,200 feet above the level of the sea, and the interior hollow is 600 +feet deep. Its external sloping surface presented a curious appearance from the +smoothness of the wide layers of tuff, which resembled a vast plastered floor. +Brattle Island is, I believe, the largest crater in the Archipelago composed of +tuff; its interior diameter is nearly a nautical mile. At present it is in a +ruined condition, consisting of little more than half a circle open to the +south; its great size is probably due, in part, to internal degradation, from +the action of the sea. +</p> + +<h3>SEGMENT OF A BASALTIC CRATER.</h3> + +<p> +(FIGURE 14. SEGMENT OF A VERY SMALL ORIFICE OF ERUPTION, on the beach of +Fresh-water Bay.) +</p> + +<p> +One side of Fresh-water Bay, in James Island, is bounded by a promontory, which +forms the last wreck of a great crater. On the beach of this promontory, a +quadrant-shaped segment of a small subordinate point of eruption stands +exposed. It consists of nine separate little streams of lava piled upon each +other; and of an irregular pinnacle, about fifteen feet high, of reddish-brown, +vesicular basalt, abounding with large crystals of glassy albite, and with +fused augite. This pinnacle, and some adjoining paps of rock on the beach, +represent the axis of the crater. The streams of lava can be followed up a +little ravine, at right angles to the coast, for between ten and fifteen yards, +where they are hidden by detritus: along the beach they are visible for nearly +eighty yards, and I do not believe that they extend much further. The three +lower streams are united to the pinnacle; and at the point of junction (as +shown in Figure 14, a rude sketch made on the spot), they are slightly arched, +as if in the act of flowing over the lip of the crater. The six upper streams +no doubt were originally united to this same column before it was worn down by +the sea. The lava of these streams is of similar composition with that of the +pinnacle, excepting that the crystals of albite appear to be more comminuted, +and the grains of fused augite are absent. Each stream is separated from the +one above it by a few inches, or at most by one or two feet in thickness, of +loose fragmentary scoriae, apparently derived from the abrasion of the streams +in passing over each other. All these streams are very remarkable from their +thinness. I carefully measured several of them; one was eight inches thick, but +was firmly coated with three inches above, and three inches below, of red +scoriaceous rock (which is the case with all the streams), making altogether a +thickness of fourteen inches: this thickness was preserved quite uniformly +along the entire length of the section. A second stream was only eight inches +thick, including both the upper and lower scoriaceous surfaces. Until examining +this section, I had not thought it possible that lava could have flowed in such +uniformly thin sheets over a surface far from smooth. These little streams +closely resemble in composition that great deluge of lava at Albemarle Island, +which likewise must have possessed a high degree of fluidity. +</p> + +<h3>PSEUDO-EXTRANEOUS, EJECTED FRAGMENTS.</h3> + +<p> +In the lava and in the scoriae of this little crater, I found several +fragments, which, from their angular form, their granular structure, their +freedom from air-cells, their brittle and burnt condition, closely resembled +those fragments of primary rocks which are occasionally ejected, as at +Ascension, from volcanoes. These fragments consist of glassy albite, much +mackled, and with very imperfect cleavages, mingled with semi-rounded grains, +having tarnished, glossy surfaces, of a steel-blue mineral. The crystals of +albite are coated by a red oxide of iron, appearing like a residual substance; +and their cleavage-planes also are sometimes separated by excessively fine +layers of this oxide, giving to the crystals the appearance of being ruled like +a glass micrometer. There was no quartz. The steel-blue mineral, which is +abundant in the pinnacle, but which disappears in the streams derived from the +pinnacle, has a fused appearance, and rarely presents even a trace of cleavage; +I obtained, however, one measurement, which proved that it was augite; and in +one other fragment, which differed from the others, in being slightly cellular, +and in gradually blending into the surrounding matrix the small grains of this +mineral were tolerably well crystallised. Although there is so wide a +difference in appearance between the lava of the little streams, and especially +of their red scoriaceous crusts, and one of these angular ejected fragments, +which at first sight might readily be mistaken for syenite, yet I believe that +the lava has originated from the melting and movement of a mass of rock of +absolutely similar composition with the fragments. Besides the specimen above +alluded to, in which we see a fragment becoming slightly cellular, and blending +into the surrounding matrix, some of the grains of the steel-blue augite also +have their surfaces becoming very finely vesicular, and passing into the nature +of the surrounding paste; other grains are throughout, in an intermediate +condition. The paste seems to consist of the augite more perfectly fused, or, +more probably, merely disturbed in its softened state by the movement of the +mass, and mingled with the oxide of iron and with finely comminuted, glassy +albite. Hence probably it is that the fused albite, which is abundant in the +pinnacle, disappears in the streams. The albite is in exactly the same state, +with the exception of most of the crystals being smaller in the lava and in the +embedded fragments; but in the fragments they appear to be less abundant: this, +however, would naturally happen from the intumescence of the augitic base, and +its consequent apparent increase in bulk. It is interesting thus to trace the +steps by which a compact granular rock becomes converted into a vesicular, +pseudo-porphyritic lava, and finally into red scoriae. The structure and +composition of the embedded fragments show that they are parts either of a mass +of primary rock which has undergone considerable change from volcanic action, +or more probably of the crust of a body of cooled and crystallised lava, which +has afterwards been broken up and re-liquified; the crust being less acted on +by the renewed heat and movement. +</p> + +<h3>CONCLUDING REMARKS ON THE TUFF-CRATERS.</h3> + +<p> +These craters, from the peculiarity of the resin-like substance which enters +largely into their composition, from their structure, their size and number, +present the most striking feature in the geology of this Archipelago. The +majority of them form either separate islets, or promontories attached to the +larger islands; and those which now stand at some little distance from the +coast are worn and breached, as if by the action of the sea. From this general +circumstance of their position, and from the small quantity of ejected ashes in +any part of the Archipelago, I am led to conclude, that the tuff has been +chiefly produced, by the grinding together of fragments of lava within active +craters, communicating with the sea. In the origin and composition of the tuff, +and in the frequent presence of a central lake of brine and of layers of salt, +these craters resemble, though on a gigantic scale, the “salses,” +or hillocks of mud, which are common in some parts of Italy and in other +countries. (D’Aubuisson “Traite de Geognosie” tome 1 page +189. I may remark, that I saw at Terceira, in the Azores, a crater of tuff or +peperino, very similar to these of the Galapagos Archipelago. From the +description given in Freycinet “Voyage,” similar ones occur at the +Sandwich Islands; and probably they are present in many other places.) Their +closer connection, however, in this Archipelago, with ordinary volcanic action, +is shown by the pools of solidified basalt, with which they are sometimes +filled up. +</p> + +<p> +It at first appears very singular, that all the craters formed of tuff have +their southern sides, either quite broken down and wholly removed, or much +lower than the other sides. I saw and received accounts of twenty-eight of +these craters; of these, twelve form separate islets (These consist of the +three Crossman Islets, the largest of which is 600 feet in height; Enchanted +Island; Gardner Island (760 feet high); Champion Island (331 feet high); +Enderby Island; Brattle Island; two islets near Indefatigable Island; and one +near James Island. A second crater near James Island (with a salt lake in its +centre) has its southern side only about twenty feet high, whilst the other +parts of the circumference are about three hundred feet in height.), and now +exist as mere crescents quite open to the south, with occasionally a few points +of rock marking their former circumference: of the remaining sixteen, some form +promontories, and others stand at a little distance inland from the shore; but +all have their southern sides either the lowest, or quite broken down. Two, +however, of the sixteen had their northern sides also low, whilst their eastern +and western sides were perfect. I did not see, or hear of, a single exception +to the rule, of these craters being broken down or low on the side, which faces +a point of the horizon between S.E. and S.W. This rule does not apply to +craters composed of lava and scoriae. The explanation is simple: at this +Archipelago, the waves from the trade-wind, and the swell propagated from the +distant parts of the open ocean, coincide in direction (which is not the case +in many parts of the Pacific), and with their united forces attack the southern +sides of all the islands; and consequently the southern slope, even when +entirely formed of hard basaltic rock, is invariably steeper than the northern +slope. As the tuff-craters are composed of a soft material, and as probably +all, or nearly all, have at some period stood immersed in the sea, we need not +wonder that they should invariably exhibit on their exposed sides the effects +of this great denuding power. Judging from the worn condition of many of these +craters, it is probable that some have been entirely washed away. As there is +no reason to suppose, that the craters formed of scoriae and lava were erupted +whilst standing in the sea, we can see why the rule does not apply to them. At +Ascension, it was shown that the mouths of the craters, which are there all of +terrestrial origin, have been affected by the trade-wind; and this same power +might here, also, aid in making the windward and exposed sides of some of the +craters originally the lowest. +</p> + +<h3>MINERALOGICAL COMPOSITION OF THE ROCKS.</h3> + +<p> +In the northern islands, the basaltic lavas seem generally to contain more +albite than they do in the southern half of the Archipelago; but almost all the +streams contain some. The albite is not unfrequently associated with olivine. I +did not observe in any specimen distinguishable crystals of hornblende or +augite; I except the fused grains in the ejected fragments, and in the pinnacle +of the little crater, above described. I did not meet with a single specimen of +true trachyte; though some of the paler lavas, when abounding with large +crystals of the harsh and glassy albite, resemble in some degree this rock; but +in every case the basis fuses into a black enamel. Beds of ashes and +far-ejected scoriae, as previously stated, are almost absent; nor did I see a +fragment of obsidian or of pumice. Von Buch believes that the absence of pumice +on Mount Etna is consequent on the feldspar being of the Labrador variety +(“Description des Isles Canaries” page 328.); if the presence of +pumice depends on the constitution of the feldspar, it is remarkable, that it +should be absent in this archipelago, and abundant in the Cordillera of South +America, in both of which regions the feldspar is of the albitic variety. Owing +to the absence of ashes, and the general indecomposable character of the lava +in this Archipelago, the islands are slowly clothed with a poor vegetation, and +the scenery has a desolate and frightful aspect. +</p> + +<h3>ELEVATION OF THE LAND.</h3> + +<p> +Proofs of the rising of the land are scanty and imperfect. At Chatham Island, I +noticed some great blocks of lava, cemented by calcareous matter, containing +recent shells; but they occurred at the height of only a few feet above +high-water mark. One of the officers gave me some fragments of shells, which he +found embedded several hundred feet above the sea, in the tuff of two craters, +distant from each other. It is possible, that these fragments may have been +carried up to their present height in an eruption of mud; but as, in one +instance, they were associated with broken oyster- shells, almost forming a +layer, it is more probable that the tuff was uplifted with the shells in mass. +The specimens are so imperfect that they can be recognised only as belonging to +recent marine genera. On Charles Island, I observed a line of great rounded +blocks, piled on the summit of a vertical cliff, at the height of fifteen feet +above the line, where the sea now acts during the heaviest gales. This +appeared, at first, good evidence in favour of the elevation of the land; but +it was quite deceptive, for I afterwards saw on an adjoining part of this same +coast, and heard from eye- witnesses, that wherever a recent stream of lava +forms a smooth inclined plane, entering the sea, the waves during gales have +the power of ROLLING UP ROUNDED blocks to a great height, above the line of +their ordinary action. As the little cliff in the foregoing case is formed by a +stream of lava, which, before being worn back, must have entered the sea with a +gently sloping surface, it is possible or rather it is probable, that the +rounded boulders, now lying on its summit, are merely the remnants of those +which had been ROLLED UP during storms to their present height. +</p> + +<h3>DIRECTION OF THE FISSURES OF ERUPTION.</h3> + +<p> +The volcanic orifices in this group cannot be considered as indiscriminately +scattered. Three great craters on Albermarle Island form a well-marked line, +extending N.W. by N. and S.E. by S. Narborough Island, and the great crater on +the rectangular projection of Albemarle Island, form a second parallel line. To +the east, Hood’s Island, and the islands and rocks between it and James +Island, form another nearly parallel line, which, when prolonged, includes +Culpepper and Wenman Islands, lying seventy miles to the north. The other +islands lying further eastward, form a less regular fourth line. Several of +these islands, and the vents on Albemarle Island, are so placed, that they +likewise fall on a set of rudely parallel lines, intersecting the former lines +at right angles; so that the principal craters appear to lie on the points +where two sets of fissures cross each other. The islands themselves, with the +exception of Albemarle Island, are not elongated in the same direction with the +lines on which they stand. The direction of these islands is nearly the same +with that which prevails in so remarkable a manner in the numerous +archipelagoes of the great Pacific Ocean. Finally, I may remark, that amongst +the Galapagos Islands there is no one dominant vent much higher than all the +others, as may be observed in many volcanic archipelagoes: the highest is the +great mound on the south- western extremity of Albemarle Island, which exceeds +by barely a thousand feet several other neighbouring craters. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap06"></a>CHAPTER VI.<br /> +TRACHYTE AND BASALT.—DISTRIBUTION OF VOLCANIC +ISLES.</h2> + +<p> +The sinking of crystals in fluid lava. Specific gravity of the constituent +parts of trachyte and of basalt, and their consequent separation. Obsidian. +Apparent non-separation of the elements of plutonic rocks. Origin of trap-dikes +in the plutonic series. Distribution of volcanic islands; their prevalence in +the great oceans. They are generally arranged in lines. The central volcanoes +of Von Buch doubtful. Volcanic islands bordering continents. Antiquity of +volcanic islands, and their elevation in mass. Eruptions on parallel lines of +fissure within the same geological period. +</p> + +<p> +ON THE SEPARATION OF THE CONSTITUENT MINERALS OF LAVA, ACCORDING TO THEIR +SPECIFIC GRAVITIES. +</p> + +<p> +One side of Fresh-water Bay, in James Island, is formed by the wreck of a large +crater, mentioned in the last chapter, of which the interior has been filled up +by a pool of basalt, about two hundred feet in thickness. This basalt is of a +grey colour, and contains many crystals of glassy albite, which become much +more numerous in the lower, scoriaceous part. This is contrary to what might +have been expected, for if the crystals had been originally disseminated in +equal numbers, the greater intumescence of this lower scoriaceous part would +have made them appear fewer in number. Von Buch has described a stream of +obsidian on the Peak of Teneriffe, in which the crystals of feldspar become +more and more numerous, as the depth or thickness increases, so that near the +lower surface of the stream the lava even resembles a primary rock. +(“Description des Isles Canaries” pages 190 and 191.) Von Buch +further states, that M. Dree, in his experiments in melting lava, found that +the crystals of feldspar always tended to precipitate themselves to the bottom +of the crucible. In these cases, I presume there can be no doubt that the +crystals sink from their weight. (In a mass of molten iron, it is found +(“Edinburgh New Philosophical Journal” volume 24 page 66) that the +substances, which have a closer affinity for oxygen than iron has, rise from +the interior of the mass to the surface. But a similar cause can hardly apply +to the separation of the crystals of these lava-streams. The cooling of the +surface of lava seems, in some cases, to have affected its composition; for +Dufrenoy (“Mem. pour servir” tome 4 page 271) found that the +interior parts of a stream near Naples contained two-thirds of a mineral which +was acted on by acids, whilst the surface consisted chiefly of a mineral +unattackable by acids.) The specific gravity of feldspar varies from 2.4 to +2.58, whilst obsidian seems commonly to be from 2.3 to 2.4; and in a fluidified +state its specific gravity would probably be less, which would facilitate the +sinking of the crystals of feldspar. (I have taken the specific gravities of +the simple minerals from Von Kobell, one of the latest and best authorities, +and of the rocks from various authorities. Obsidian, according to Phillips, is +2.35; and Jameson says it never exceeds 2.4; but a specimen from Ascension, +weighed by myself, was 2.42.) At James Island, the crystals of albite, though +no doubt of less weight than the grey basalt, in the parts where compact, might +easily be of greater specific gravity than the scoriaceous mass, formed of +melted lava and bubbles of heated gas. +</p> + +<p> +The sinking of crystals through a viscid substance like molten rock, as is +unequivocally shown to have been the case in the experiments of M. Dree, is +worthy of further consideration, as throwing light on the separation of the +trachytic and basaltic series of lavas. Mr. P. Scrope has speculated on this +subject; but he does not seem to have been aware of any positive facts, such as +those above given; and he has overlooked one very necessary element, as it +appears to me, in the phenomenon—namely, the existence of either the +lighter or heavier mineral in globules or in crystals. In a substance of +imperfect fluidity, like molten rock, it is hardly credible, that the separate, +infinitely small atoms, whether of feldspar, augite, or of any other mineral, +would have power from their slightly different gravities to overcome the +friction caused by their movement; but if the atoms of any one of these +minerals became, whilst the others remained fluid, united into crystals or +granules, it is easy to perceive that from the lessened friction, their sinking +or floating power would be greatly increased. On the other hand, if all the +minerals became granulated at the same time, it is scarcely possible, from +their mutual resistance, that any separation could take place. A valuable, +practical discovery, illustrating the effect of the granulation of one element +in a fluid mass, in aiding its separation, has lately been made: when lead +containing a small proportion of silver, is constantly stirred whilst cooling, +it becomes granulated, and the grains of imperfect crystals of nearly pure lead +sink to the bottom, leaving a residue of melted metal much richer in silver; +whereas if the mixture be left undisturbed, although kept fluid for a length of +time, the two metals show no signs of separating. (A full and interesting +account of this discovery, by Mr. Pattinson, was read before the British +Association in September 1838. In some alloys, according to Turner +“Chemistry” page 210, the heaviest metal sinks, and it appears that +this takes place whilst both metals are fluid. Where there is a considerable +difference in gravity, as between iron and the slag formed during the fusion of +the ore, we need not be surprised at the atoms separating, without either +substance being granulated.) The sole use of the stirring seems to be, the +formation of detached granules. The specific gravity of silver is 10.4, and of +lead 11.35: the granulated lead, which sinks, is never absolutely pure, and the +residual fluid metal contains, when richest, only 1/119 part of silver. As the +difference in specific gravity, caused by the different proportions of the two +metals, is so exceedingly small, the separation is probably aided in a great +degree by the difference in gravity between the lead, when granular though +still hot, and when fluid. +</p> + +<p> +In a body of liquified volcanic rock, left for some time without any violent +disturbance, we might expect, in accordance with the above facts, that if one +of the constituent minerals became aggregated into crystals or granules, or had +been enveloped in this state from some previously existing mass, such crystals +or granules would rise or sink, according to their specific gravity. Now we +have plain evidence of crystals being embedded in many lavas, whilst the paste +or basis has continued fluid. I need only refer, as instances, to the several, +great, pseudo-porphyritic streams at the Galapagos Islands, and to the +trachytic streams in many parts of the world, in which we find crystals of +feldspar bent and broken by the movement of the surrounding, semi-fluid matter. +Lavas are chiefly composed of three varieties of feldspar, varying in specific +gravity from 2.4 to 2.74; of hornblende and augite, varying from 3.0 to 3.4; of +olivine, varying from 3.3 to 3.4; and lastly, of oxides of iron, with specific +gravities from 4.8 to 5.2. Hence crystals of feldspar, enveloped in a mass of +liquified, but not highly vesicular lava, would tend to rise to the upper +parts; and crystals or granules of the other minerals, thus enveloped, would +tend to sink. We ought not, however, to expect any perfect degree of separation +in such viscid materials. Trachyte, which consists chiefly of feldspar, with +some hornblende and oxide of iron, has a specific gravity of about 2.45; whilst +basalt, composed chiefly of augite and feldspar, often with much iron and +olivine, has a gravity of about 3.0. (Trachyte from Java was found by Von Buch +to be 2.47; from Auvergne, by De la Beche, it was 2.42; from Ascension, by +myself, it was 2.42. Jameson and other authors give to basalt a specific +gravity of 3.0; but specimens from Auvergne were found, by De la Beche, to be +only 2.78; and from the Giant’s Causeway, to be 2.91.) Accordingly we +find, that where both trachytic and basaltic streams have proceeded from the +same orifice, the trachytic streams have generally been first erupted owing, as +we must suppose, to the molten lava of this series having accumulated in the +upper parts of the volcanic focus. This order of eruption has been observed by +Beudant, Scrope, and by other authors; three instances, also, have been given +in this volume. As the later eruptions, however, from most volcanic mountains, +burst through their basal parts, owing to the increased height and weight of +the internal column of molten rock, we see why, in most cases, only the lower +flanks of the central, trachytic masses, are enveloped by basaltic streams. The +separation of the ingredients of a mass of lava, would, perhaps, sometimes take +place within the body of a volcanic mountain, if lofty and of great dimensions, +instead of within the underground focus; in which case, trachytic streams might +be poured forth, almost contemporaneously, or at short recurrent intervals, +from its summit, and basaltic streams from its base: this seems to have taken +place at Teneriffe. (Consult Von Buch’s well-known and admirable +“Description Physique” of this island, which might serve as a model +of descriptive geology.) I need only further remark, that from violent +disturbances the separation of the two series, even under otherwise favourable +conditions, would naturally often be prevented, and likewise their usual order +of eruption be inverted. From the high degree of fluidity of most basaltic +lavas, these perhaps, alone, would in many cases reach the surface. +</p> + +<p> +As we have seen that crystals of feldspar, in the instance described by Von +Buch, sink in obsidian, in accordance with their known greater specific +gravity, we might expect to find in every trachytic district, where obsidian +has flowed as lava, that it had proceeded from the upper or highest orifices. +This, according to Von Buch, holds good in a remarkable manner both at the +Lipari Islands and on the Peak of Teneriffe; at this latter place obsidian has +never flowed from a less height than 9,200 feet. Obsidian, also, appears to +have been erupted from the loftiest peaks of the Peruvian Cordillera. I will +only further observe, that the specific gravity of quartz varies from 2.6 to +2.8; and therefore, that when present in a volcanic focus, it would not tend to +sink with the basaltic bases; and this, perhaps, explains the frequent +presence, and the abundance of this mineral, in the lavas of the trachytic +series, as observed in previous parts of this volume. +</p> + +<p> +An objection to the foregoing theory will, perhaps, be drawn from the plutonic +rocks not being separated into two evidently distinct series, of different +specific gravities; although, like the volcanic, they have been liquified. In +answer, it may first be remarked, that we have no evidence of the atoms of any +one of the constituent minerals in the plutonic series having been aggregated, +whilst the others remained fluid, which we have endeavoured to show is an +almost necessary condition of their separation; on the contrary, the crystals +have generally impressed each other with their forms. (The crystalline paste of +phonolite is frequently penetrated by long needles of hornblende; from which it +appears that the hornblende, though the more fusible mineral, has crystallised +before, or at the same time with a more refractory substance. Phonolite, as far +as my observations serve, in every instance appears to be an injected rock, +like those of the plutonic series; hence probably, like these latter, it has +generally been cooled without repeated and violent disturbances. Those +geologists who have doubted whether granite could have been formed by igneous +liquefaction, because minerals of different degrees of fusibility impress each +other with their forms, could not have been aware of the fact of crystallised +hornblende penetrating phonolite, a rock undoubtedly of igneous origin. The +viscidity, which it is now known, that both feldspar and quartz retain at a +temperature much below their points of fusion, easily explains their mutual +impressment. Consult on this subject Mr. Horner’s paper on Bonn +“Geolog. Transact.” volume 4 page 439; and +“L’Institut” with respect to quartz 1839 page 161.) +</p> + +<p> +In the second place, the perfect tranquillity, under which it is probable that +the plutonic masses, buried at profound depths, have cooled, would, most +likely, be highly unfavourable to the separation of their constituent minerals; +for, if the attractive force, which during the progressive cooling draws +together the molecules of the different minerals, has power sufficient to keep +them together, the friction between such half-formed crystals or pasty globules +would effectually prevent the heavier ones from sinking, or the lighter ones +from rising. On the other hand, a small amount of disturbance, which would +probably occur in most volcanic foci, and which we have seen does not prevent +the separation of granules of lead from a mixture of molten lead and silver, or +crystals of feldspar from streams of lava, by breaking and dissolving the less +perfectly formed globules, would permit the more perfect and therefore unbroken +crystals, to sink or rise, according to their specific gravity. +</p> + +<p> +Although in plutonic rocks two distinct species, corresponding to the trachytic +and basaltic series, do not exist, I much suspect that a certain amount of +separation of their constituent parts has often taken place. I suspect this +from having observed how frequently dikes of greenstone and basalt intersect +widely extended formations of granite and the allied metamorphic rocks. I have +never examined a district in an extensive granitic region without discovering +dikes; I may instance the numerous trap-dikes, in several districts of Brazil, +Chile, and Australia, and at the Cape of Good Hope: many dikes likewise occur +in the great granitic tracts of India, in the north of Europe, and in other +countries. Whence, then, has the greenstone and basalt, forming these dikes, +come? Are we to suppose, like some of the elder geologists, that a zone of trap +is uniformly spread out beneath the granitic series, which composes, as far as +we know, the foundations of the earth’s crust? Is it not more probable, +that these dikes have been formed by fissures penetrating into partially cooled +rocks of the granitic and metamorphic series, and by their more fluid parts, +consisting chiefly of hornblende, oozing out, and being sucked into such +fissures? At Bahia, in Brazil, in a district composed of gneiss and primitive +greenstone, I saw many dikes, of a dark augitic (for one crystal certainly was +of this mineral) or hornblendic rock, which, as several appearances clearly +proved, either had been formed before the surrounding mass had become solid, or +had together with it been afterwards thoroughly softened. (Portions of these +dikes have been broken off, and are now surrounded by the primary rocks, with +their laminae conformably winding round them. Dr. Hubbard also +(“Silliman’s Journal” volume 34 page 119), has described an +interlacement of trap-veins in the granite of the White Mountains, which he +thinks must have been formed when both rocks were soft.) On both sides of one +of these dikes, the gneiss was penetrated, to the distance of several yards, by +numerous, curvilinear threads or streaks of dark matter, which resembled in +form clouds of the class called cirrhi- comae; some few of these threads could +be traced to their junction with the dike. When examining them, I doubted +whether such hair-like and curvilinear veins could have been injected, and I +now suspect, that instead of having been injected from the dike, they were its +feeders. If the foregoing views of the origin of trap-dikes in widely extended +granitic regions far from rocks of any other formation, be admitted as +probable, we may further admit, in the case of a great body of plutonic rock, +being impelled by repeated movements into the axis of a mountain-chain, that +its more liquid constituent parts might drain into deep and unseen abysses; +afterwards, perhaps, to be brought to the surface under the form, either of +injected masses of greenstone and augitic porphyry, or of basaltic eruptions. +(Mr. Phillips “Lardner’s Encyclop.” volume 2 page 115 quotes +Von Buch’s statement, that augitic porphyry ranges parallel to, and is +found constantly at the base of, great chains of mountains. Humboldt, also, has +remarked the frequent occurrence of trap-rock, in a similar position; of which +fact I have observed many examples at the foot of the Chilian Cordillera. The +existence of granite in the axes of great mountain chains is always probable, +and I am tempted to suppose, that the laterally injected masses of augitic +porphyry and of trap, bear nearly the same relation to the granitic axes which +basaltic lavas bear to the central trachytic masses, round the flanks of which +they have so frequently been erupted.) Much of the difficulty which geologists +have experienced when they have compared the composition of volcanic with +plutonic formations, will, I think, be removed, if we may believe that most +plutonic masses have been, to a certain extent, drained of those comparatively +weighty and easily liquified elements, which compose the trappean and basaltic +series of rocks. +</p> + +<h3>ON THE DISTRIBUTION OF VOLCANIC ISLANDS.</h3> + +<p> +During my investigations on coral-reefs, I had occasion to consult the works of +many voyagers, and I was invariably struck with the fact, that with rare +exceptions, the innumerable islands scattered throughout the Pacific, Indian, +and Atlantic Oceans, were composed either of volcanic, or of modern +coral-rocks. It would be tedious to give a long catalogue of all the volcanic +islands; but the exceptions which I have found are easily enumerated: in the +Atlantic, we have St. Paul’s Rock, described in this volume, and the +Falkland Islands, composed of quartz and clay-slate; but these latter islands +are of considerable size, and lie not very far from the South American coast +(Judging from Forster’s imperfect observation, perhaps Georgia is not +volcanic. Dr. Allan is my informant with regard to the Seychelles. I do not +know of what formation Rodriguez, in the Indian Ocean, is composed.): in the +Indian Ocean, the Seychelles (situated in a line prolonged from Madagascar) +consist of granite and quartz: in the Pacific Ocean, New Caledonia, an island +of large size, belongs (as far as is known) to the primary class. New Zealand, +which contains much volcanic rock and some active volcanoes, from its size +cannot be classed with the small islands, which we are now considering. The +presence of a small quantity of non-volcanic rock, as of clay-slate on three of +the Azores (This is stated on the authority of Count V. de Bedemar, with +respect to Flores and Graciosa (Charlsworth “Magazine of Nat. +Hist.” volume 1 page 557). St. Maria has no volcanic rock, according to +Captain Boyd (Von Buch “Descript.” page 365). Chatham Island has +been described by Dr. Dieffenbach in the “Geographical Journal” +1841 page 201. As yet we have received only imperfect notices on Kerguelen +Land, from the Antarctic Expedition.), or of tertiary limestone at Madeira, or +of clay-slate at Chatham Island in the Pacific, or of lignite at Kerguelen +Land, ought not to exclude such islands or archipelagoes, if formed chiefly of +erupted matter, from the volcanic class. +</p> + +<p> +The composition of the numerous islands scattered through the great oceans +being with such rare exceptions volcanic, is evidently an extension of that +law, and the effect of those same causes, whether chemical or mechanical, from +which it results, that a vast majority of the volcanoes now in action stand +either as islands in the sea, or near its shores. This fact of the +ocean-islands being so generally volcanic is also interesting in relation to +the nature of the mountain-chains on our continents, which are comparatively +seldom volcanic; and yet we are led to suppose that where our continents now +stand an ocean once extended. Do volcanic eruptions, we may ask, reach the +surface more readily through fissures formed during the first stages of the +conversion of the bed of the ocean into a tract of land? +</p> + +<p> +Looking at the charts of the numerous volcanic archipelagoes, we see that the +islands are generally arranged either in single, double, or triple rows, in +lines which are frequently curved in a slight degree. (Professors William and +Henry Darwin Rogers have lately insisted much, in a memoir read before the +American Association, on the regularly curved lines of elevation in parts of +the Appalachian range.) Each separate island is either rounded, or more +generally elongated in the same direction with the group in which it stands, +but sometimes transversely to it. Some of the groups which are not much +elongated present little symmetry in their forms; M. Virlet (“Bulletin de +la Soc. Geolog.” tome 3 page 110.) states that this is the case with the +Grecian Archipelago: in such groups I suspect (for I am aware how easy it is to +deceive oneself on these points), that the vents are generally arranged on one +line, or on a set of short parallel lines, intersecting at nearly right angles +another line, or set of lines. The Galapagos Archipelago offers an example of +this structure, for most of the islands and the chief orifices on the largest +island are so grouped as to fall on a set of lines ranging about N.W. by N., +and on another set ranging about W.S.W.: in the Canary Archipelago we have a +simpler structure of the same kind: in the Cape de Verde group, which appears +to be the least symmetrical of any oceanic volcanic archipelago, a N.W. and +S.E. line formed by several islands, if prolonged, would intersect at right +angles a curved line, on which the remaining islands are placed. +</p> + +<p> +Von Buch (“Description des Isles Canaries” page 324.) has classed +all volcanoes under two heads, namely, CENTRAL VOLCANOES, round which numerous +eruptions have taken place on all sides, in a manner almost regular, and +VOLCANIC CHAINS. In the examples given of the first class, as far as position +is concerned, I can see no grounds for their being called +“central;” and the evidence of any difference in mineralogical +nature between CENTRAL VOLCANOES and VOLCANIC CHAINS appears slight. No doubt +some one island in most small volcanic archipelagoes is apt to be considerably +higher than the others; and in a similar manner, whatever the cause may be, +that on the same island one vent is generally higher than all the others. Von +Buch does not include in his class of volcanic chains small archipelagoes, in +which the islands are admitted by him, as at the Azores, to be arranged in +lines; but when viewing on a map of the world how perfect a series exists from +a few volcanic islands placed in a row to a train of linear archipelagoes +following each other in a straight line, and so on to a great wall like the +Cordillera of America, it is difficult to believe that there exists any +essential difference between short and long volcanic chains. Von Buch (Idem +page 393.) states that his volcanic chains surmount, or are closely connected +with, mountain-ranges of primary formation: but if trains of linear +archipelagoes are, in the course of time, by the long- continued action of the +elevatory and volcanic forces, converted into mountain-ranges, it would +naturally result that the inferior primary rocks would often be uplifted and +brought into view. +</p> + +<p> +Some authors have remarked that volcanic islands occur scattered, though at +very unequal distances, along the shores of the great continents, as if in some +measure connected with them. In the case of Juan Fernandez, situated 330 miles +from the coast of Chile, there was undoubtedly a connection between the +volcanic forces acting under this island and under the continent, as was shown +during the earthquake of 1835. The islands, moreover, of some of the small +volcanic groups which thus border continents, are placed in lines, related to +those along which the adjoining shores of the continents trend; I may instance +the lines of intersection at the Galapagos, and at the Cape de Verde +Archipelagoes, and the best marked line of the Canary Islands. If these facts +be not merely accidental, we see that many scattered volcanic islands and small +groups are related not only by proximity, but in the direction of the fissures +of eruption to the neighbouring continents—a relation, which Von Buch +considers, characteristic of his great volcanic chains. +</p> + +<p> +In volcanic archipelagoes, the orifices are seldom in activity on more than one +island at a time; and the greater eruptions usually recur only after long +intervals. Observing the number of craters, that are usually found on each +island of a group, and the vast amount of matter which has been erupted from +them, one is led to attribute a high antiquity even to those groups, which +appear, like the Galapagos, to be of comparatively recent origin. This +conclusion accords with the prodigious amount of degradation, by the slow +action of the sea, which their originally sloping coasts must have suffered, +when they are worn back, as is so often the case, into grand precipices. We +ought not, however, to suppose, in hardly any instance, that the whole body of +matter, forming a volcanic island, has been erupted at the level, on which it +now stands: the number of dikes, which seem invariably to intersect the +interior parts of every volcano, show, on the principles explained by M. Elie +de Beaumont, that the whole mass has been uplifted and fissured. A connection, +moreover, between volcanic eruptions and contemporaneous elevations in mass +has, I think, been shown to exist in my work on Coral-Reefs, both from the +frequent presence of upraised organic remains, and from the structure of the +accompanying coral-reefs. (A similar conclusion is forced on us, by the +phenomena, which accompanied the earthquake of 1835, at Concepcion, and which +are detailed in my paper (volume 5 page 601) in the “Geological +Transactions.”) Finally, I may remark, that in the same Archipelago, +eruptions have taken place within the historical period on more than one of the +parallel lines of fissure: thus, at the Galapagos Archipelago, eruptions have +taken place from a vent on Narborough Island, and from one on Albemarle Island, +which vents do not fall on the same line; at the Canary Islands, eruptions have +taken place in Teneriffe and Lanzarote; and at the Azores, on the three +parallel lines of Pico, St. Jorge, and Terceira. Believing that a mountain-axis +differs essentially from a volcano, only in plutonic rocks having been +injected, instead of volcanic matter having been ejected, this appears to me an +interesting circumstance; for we may infer from it as probable, that in the +elevation of a mountain-chain, two or more of the parallel lines forming it may +be upraised and injected within the same geological period. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap07"></a>CHAPTER VII.<br /> +AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE.</h2> + +<p> +New South Wales. Sandstone formation. Embedded pseudo-fragments of shale. +Stratification. Current-cleavage. Great valleys. Van Diemen’s Land. +Palaeozoic formation. Newer formation with volcanic rocks. Travertin with +leaves of extinct plants. Elevation of the land. New Zealand. King +George’s Sound. Superficial ferruginous beds. Superficial calcareous +deposits, with casts of branches. Their origin from drifted particles of shells +and corals. Their extent. Cape of Good Hope. Junction of the granite and +clay-slate. Sandstone formation. +</p> + +<p> +The “Beagle,” in her homeward voyage, touched at New Zealand, +Australia, Van Diemen’s Land, and the Cape of Good Hope. In order to +confine the Third Part of these Geological Observations to South America, I +will here briefly describe all that I observed at these places worthy of the +attention of geologists. +</p> + +<h3>NEW SOUTH WALES.</h3> + +<p> +My opportunities of observation consisted of a ride of ninety geographical +miles to Bathurst, in a W.N.W. direction from Sydney. The first thirty miles +from the coast passes over a sandstone country, broken up in many places by +trap-rocks, and separated by a bold escarpment overhanging the river Nepean, +from the great sandstone platform of the Blue Mountains. This upper platform is +1,000 feet high at the edge of the escarpment, and rises in a distance of +twenty-five miles to between three and four thousand feet above the level of +the sea. At this distance the road descends to a country rather less elevated, +and composed in chief part of primary rocks. There is much granite, in one part +passing into a red porphyry with octagonal crystals of quartz, and intersected +in some places by trap-dikes. Near the Downs of Bathurst I passed over much +pale-brown, glossy clay-slate, with the shattered laminae running north and +south; I mention this fact, because Captain King informs me that, in the +country a hundred miles southward, near Lake George, the mica-slate ranges so +invariably north and south that the inhabitants take advantage of it in finding +their way through the forests. +</p> + +<p> +The sandstone of the Blue Mountains is at least 1,200 feet thick, and in some +parts is apparently of greater thickness; it consists of small grains of +quartz, cemented by white earthy matter, and it abounds with ferruginous veins. +The lower beds sometimes alternate with shales and coal: at Wolgan I found in +carbonaceous shale leaves of the Glossopteris Brownii, a fern which so +frequently accompanies the coal of Australia. The sandstone contains pebbles of +quartz; and these generally increase in number and size (seldom, however, +exceeding an inch or two in diameter) in the upper beds: I observed a similar +circumstance in the grand sandstone formation at the Cape of Good Hope. On the +South American coast, where tertiary and supra- tertiary beds have been +extensively elevated, I repeatedly noticed that the uppermost beds were formed +of coarser materials than the lower: this appears to indicate that, as the sea +became shallower, the force of the waves or currents increased. On the lower +platform, however, between the Blue Mountains and the coast, I observed that +the upper beds of the sandstone frequently passed into argillaceous +shale,—the effect, probably, of this lower space having been protected +from strong currents during its elevation. The sandstone of the Blue Mountains +evidently having been of mechanical origin, and not having suffered any +metamorphic action, I was surprised at observing that, in some specimens, +nearly all the grains of quartz were so perfectly crystallised with brilliant +facets that they evidently had not in their PRESENT form been aggregated in any +previously existing rock. (I have lately seen, in a paper by Smith (the father +of English geologists), in the “Magazine of Natural History,” that +the grains of quartz in the millstone grit of England are often crystallised. +Sir David Brewster, in a paper read before the British Association, 1840, +states, that in old decomposed glass, the silex and metals separate into +concentric rings, and that the silex regains its crystalline structure, as is +shown by its action on light.) It is difficult to imagine how these crystals +could have been formed; one can hardly believe that they were separately +precipitated in their present crystallised state. Is it possible that rounded +grains of quartz may have been acted on by a fluid corroding their surfaces, +and depositing on them fresh silica? I may remark that, in the sandstone +formation of the Cape of Good Hope, it is evident that silica has been +profusely deposited from aqueous solution. +</p> + +<p> +In several parts of the sandstone I noticed patches of shale which might at the +first glance have been mistaken for extraneous fragments; their horizontal +laminae, however, being parallel with those of the sandstone, showed that they +were the remnants of thin, continuous beds. One such fragment (probably the +section of a long narrow strip) seen in the face of a cliff, was of greater +vertical thickness than breadth, which proves that this bed of shale must have +been in some slight degree consolidated, after having been deposited, and +before being worn away by the currents. Each patch of the shale shows, also, +how slowly many of the successive layers of sandstone were deposited. These +pseudo-fragments of shale will perhaps explain, in some cases, the origin of +apparently extraneous fragments in crystalline metamorphic rocks. I mention +this, because I found near Rio de Janeiro a well-defined angular fragment, +seven yards long by two yards in breadth, of gneiss containing garnets and mica +in layers, enclosed in the ordinary, stratified, porphyritic gneiss of the +country. The laminae of the fragment and of the surrounding matrix ran in +exactly the same direction, but they dipped at different angles. I do not wish +to affirm that this singular fragment (a solitary case, as far as I know) was +originally deposited in a layer, like the shale in the Blue Mountains, between +the strata of the porphyritic gneiss, before they were metamorphosed; but there +is sufficient analogy between the two cases to render such an explanation +possible. +</p> + +<h3>STRATIFICATION OF THE ESCARPMENT.</h3> + +<p> +The strata of the Blue Mountains appear to the eye horizontal; but they +probably have a similar inclination with the surface of the platform, which +slopes from the west towards the escarpment over the Nepean, at an angle of one +degree, or of one hundred feet in a mile. (This is stated on the authority of +Sir T. Mitchell in “Travels” volume 2 page 357.) The strata of the +escarpment dip almost conformably with its steeply inclined face, and with so +much regularity, that they appear as if thrown into their present position; but +on a more careful examination, they are seen to thicken and to thin out, and in +the upper part to be succeeded and almost capped by horizontal beds. These +appearances render it probable, that we here see an original escarpment, not +formed by the sea having eaten back into the strata, but by the strata having +originally extended only thus far. Those who have been in the habit of +examining accurate charts of sea-coasts, where sediment is accumulating, will +be aware, that the surfaces of the banks thus formed, generally slope from the +coast very gently towards a certain line in the offing, beyond which the depth +in most cases suddenly becomes great. I may instance the great banks of +sediment within the West Indian Archipelago (I have described these very +curious banks in the Appendix to my volume on the structure of Coral-Reefs. I +have ascertained the inclination of the edges of the banks, from information +given me by Captain B. Allen, one of the surveyors, and by carefully measuring +the horizontal distances between the last sounding on the bank and the first in +the deep water. Widely extended banks in all parts of the West Indies have the +same general form of surface.), which terminate in submarine slopes, inclined +at angles of between thirty and forty degrees, and sometimes even at more than +forty degrees: every one knows how steep such a slope would appear on the land. +Banks of this nature, if uplifted, would probably have nearly the same external +form as the platform of the Blue Mountains, where it abruptly terminates over +the Nepean. +</p> + +<h3>CURRENT-CLEAVAGE.</h3> + +<p> +The strata of sandstone in the low coast country, and likewise on the Blue +Mountains, are often divided by cross or current laminae, which dip in +different directions, and frequently at an angle of forty-five degrees. Most +authors have attributed these cross layers to successive small accumulations on +an inclined surface; but from a careful examination in some parts of the New +Red Sandstone of England, I believe that such layers generally form parts of a +series of curves, like gigantic tidal ripples, the tops of which have since +been cut off, either by nearly horizontal layers, or by another set of great +ripples, the folds of which do not exactly coincide with those below them. It +is well-known to surveyors that mud and sand are disturbed during storms at +considerable depths, at least from three hundred to four hundred and fifty feet +(See Martin White on “Soundings in the British Channel” pages 4 and +166.), so that the nature of the bottom even becomes temporarily changed; the +bottom, also, at a depth between sixty and seventy feet, has been observed to +be broadly rippled. (M. Siau on the “Action of Waves” “Edin. +New Phil. Journ.” volume 31 page 245.) One may, therefore, be allowed to +suspect, from the appearance just mentioned in the New Red Sandstone, that at +greater depths, the bed of the ocean is heaped up during gales into great +ripple-like furrows and depressions, which are afterwards cut off by the +currents during more tranquil weather, and again furrowed during gales. +</p> + +<h3>VALLEYS IN THE SANDSTONE PLATFORMS.</h3> + +<p> +The grand valleys, by which the Blue Mountains and the other sandstone +platforms of this part of Australia are penetrated, and which long offered an +insuperable obstacle to the attempts of the most enterprising colonist to reach +the interior country, form the most striking feature in the geology of New +South Wales. They are of grand dimensions, and are bordered by continuous links +of lofty cliffs. It is not easy to conceive a more magnificent spectacle, than +is presented to a person walking on the summit- plains, when without any notice +he arrives at the brink of one of these cliffs, which are so perpendicular, +that he can strike with a stone (as I have tried) the trees growing, at the +depth of between one thousand and one thousand five hundred feet below him; on +both hands he sees headland beyond headland of the receding line of cliff, and +on the opposite side of the valley, often at the distance of several miles, he +beholds another line rising up to the same height with that on which he stands, +and formed of the same horizontal strata of pale sandstone. The bottoms of +these valleys are moderately level, and the fall of the rivers flowing in them, +according to Sir T. Mitchell, is gentle. The main valleys often send into the +platform great baylike arms, which expand at their upper ends; and on the other +hand, the platform often sends promontories into the valley, and even leaves in +them great, almost insulated, masses. So continuous are the bounding lines of +cliff, that to descend into some of these valleys, it is necessary to go round +twenty miles; and into others, the surveyors have only lately penetrated, and +the colonists have not yet been able to drive in their cattle. But the most +remarkable point of structure in these valleys, is, that although several miles +wide in their upper parts, they generally contract towards their mouths to such +a degree as to become impassable. The Surveyor-General, Sir T. Mitchell, in +vain endeavoured, first on foot and then by crawling between the great fallen +fragments of sandstone, to ascend through the gorge by which the river Grose +joins the Nepean (“Travels in Australia” volume 1 page 154.—I +must express my obligation to Sir T. Mitchell for several interesting personal +communications on the subject of these great valleys of New South Wales.); yet +the valley of the Grose in its upper part, as I saw, forms a magnificent basin +some miles in width, and is on all sides surrounded by cliffs, the summits of +which are believed to be nowhere less than 3,000 feet above the level of the +sea. When cattle are driven into the valley of the Wolgan by a path (which I +descended) partly cut by the colonists, they cannot escape; for this valley is +in every other part surrounded by perpendicular cliffs, and eight miles lower +down, it contracts, from an average width of half a mile, to a mere chasm +impassable to man or beast. Sir T. Mitchell states, that the great valley of +the Cox river with all its branches contracts, where it unites with the Nepean, +into a gorge 2,200 yards wide, and about one thousand feet in depth. (Idem +volume 2 page 358.) Other similar cases might have been added. +</p> + +<p> +The first impression, from seeing the correspondence of the horizontal strata, +on each side of these valleys and great amphitheatre-like depressions, is that +they have been in chief part hollowed out, like other valleys, by aqueous +erosion; but when one reflects on the enormous amount of stone, which on this +view must have been removed, in most of the above cases through mere gorges or +chasms, one is led to ask whether these spaces may not have subsided. But +considering the form of the irregularly branching valleys, and of the narrow +promontories, projecting into them from the platforms, we are compelled to +abandon this notion. To attribute these hollows to alluvial action, would be +preposterous; nor does the drainage from the summit-level always fall, as I +remarked near the Weatherboard, into the head of these valleys, but into one +side of their bay-like recesses. Some of the inhabitants remarked to me, that +they never viewed one of these baylike recesses, with the headlands receding on +both hands, without being struck with their resemblance to a bold sea-coast. +This is certainly the case; moreover, the numerous fine harbours, with their +widely branching arms, on the present coast of New South Wales, which are +generally connected with the sea by a narrow mouth, from one mile to a quarter +of a mile in width, passing through the sandstone coast-cliffs, present a +likeness, though on a miniature scale, to the great valleys of the interior. +But then immediately occurs the startling difficulty, why has the sea worn out +these great, though circumscribed, depressions on a wide platform, and left +mere gorges, through which the whole vast amount of triturated matter must have +been carried away? The only light I can throw on this enigma, is by showing +that banks appear to be forming in some seas of the most irregular forms, and +that the sides of such banks are so steep (as before stated) that a +comparatively small amount of subsequent erosion would form them into cliffs: +that the waves have power to form high and precipitous cliffs, even in +landlocked harbours, I have observed in many parts of South America. In the Red +Sea, banks with an extremely irregular outline and composed of sediment, are +penetrated by the most singularly shaped creeks with narrow mouths: this is +likewise the case, though on a larger scale, with the Bahama Banks. Such banks, +I have been led to suppose, have been formed by currents heaping sediment on an +irregular bottom. (See the “Appendix” to the Part on Coral-Reefs. +The fact of the sea heaping up mud round a submarine nucleus, is worthy of the +notice of geologists: for outlyers of the same composition with the coast banks +are thus formed; and these, if upheaved and worn into cliffs, would naturally +be thought to have been once connected together.) That in some cases, the sea, +instead of spreading out sediment in a uniform sheet, heaps it round submarine +rocks and islands, it is hardly possible to doubt, after having examined the +charts of the West Indies. To apply these ideas to the sandstone platforms of +New South Wales, I imagine that the strata might have been heaped on an +irregular bottom by the action of strong currents, and of the undulations of an +open sea; and that the valley-like spaces thus left unfilled might, during a +slow elevation of the land, have had their steeply sloping flanks worn into +cliffs; the worn-down sandstone being removed, either at the time when the +narrow gorges were cut by the retreating sea, or subsequently by alluvial +action. +</p> + +<h3>VAN DIEMEN’S LAND.</h3> + +<p> +The southern part of this island is mainly formed of mountains of greenstone, +which often assumes a syenitic character, and contains much hypersthene. These +mountains, in their lower half, are generally encased by strata containing +numerous small corals and some shells. These shells have been examined by Mr. +G.B. Sowerby, and have been described by him: they consist of two species of +Producta, and of six of Spirifera; two of these, namely, P. rugata and S. +rotundata, resemble, as far as their imperfect condition allows of comparison, +British mountain-limestone shells. Mr. Lonsdale has had the kindness to examine +the corals; they consist of six undescribed species, belonging to three genera. +Species of these genera occur in the Silurian, Devonian, and Carboniferous +strata of Europe. Mr. Lonsdale remarks, that all these fossils have undoubtedly +a Palaeozoic character, and that probably they correspond in age to a division +of the system above the Silurian formations. +</p> + +<p> +The strata containing these remains are singular from the extreme variability +of their mineralogical composition. Every intermediate form is present, between +flinty-slate, clay-slate passing into grey wacke, pure limestone, sandstone, +and porcellanic rock; and some of the beds can only be described as composed of +a siliceo-calcareo-clay-slate. The formation, as far as I could judge, is at +least a thousand feet in thickness: the upper few hundred feet usually consist +of a siliceous sandstone, containing pebbles and no organic remains; the +inferior strata, of which a pale flinty slate is perhaps the most abundant, are +the most variable; and these chiefly abound with the remains. Between two beds +of hard crystalline limestone, near Newtown, a layer of white soft calcareous +matter is quarried, and is used for whitewashing houses. From information given +to me by Mr. Frankland, the Surveyor-General, it appears that this Palaeozoic +formation is found in different parts of the whole island; from the same +authority, I may add, that on the north-eastern coast and in Bass’ +Straits primary rocks extensively occur. +</p> + +<p> +The shores of Storm Bay are skirted, to the height of a few hundred feet, by +strata of sandstone, containing pebbles of the formation just described, with +its characteristic fossils, and therefore belonging to a subsequent age. These +strata of sandstone often pass into shale, and alternate with layers of impure +coal; they have in many places been violently disturbed. Near Hobart Town, I +observed one dike, nearly a hundred yards in width, on one side of which the +strata were tilted at an angle of 60 degrees, and on the other they were in +some parts vertical, and had been altered by the effects of the heat. On the +west side of Storm Bay, I found these strata capped by streams of basaltic lava +with olivine; and close by there was a mass of brecciated scoriae, containing +pebbles of lava, which probably marks the place of an ancient submarine crater. +Two of these streams of basalt were separated from each other by a layer of +argillaceous wacke, which could be traced passing into partially altered +scoriae. The wacke contained numerous rounded grains of a soft, grass-green +mineral, with a waxy lustre, and translucent on its edges: under the blowpipe +it instantly blackened, and the points fused into a strongly magnetic, black +enamel. In these characters, it resembles those masses of decomposed olivine, +described at St. Jago in the Cape de Verde group; and I should have thought +that it had thus originated, had I not found a similar substance, in +cylindrical threads, within the cells of the vesicular basalt,—a state +under which olivine never appears; this substance, I believe, would be classed +as bole by mineralogists. (Chlorophaeite, described by Dr. MacCulloch +(“Western Islands” volume 1 page 504) as occurring in a basaltic +amygdaloid, differs from this substance, in remaining unchanged before the +blowpipe, and in blackening from exposure to the air. May we suppose that +olivine, in undergoing the remarkable change described at St. Jago, passes +through several states?) +</p> + +<h3>TRAVERTIN WITH EXTINCT PLANTS.</h3> + +<p> +Behind Hobart Town there is a small quarry of a hard travertin, the lower +strata of which abound with distinct impressions of leaves. Mr. Robert Brown +has had the kindness to look at my specimens, and he informed me that there are +four or five kinds, none of which he recognises as belonging to existing +species. The most remarkable leaf is palmate, like that of a fan- palm, and no +plant having leaves of this structure has hitherto been discovered in Van +Diemen’s Land. The other leaves do not resemble the most usual form of +the Eucalyptus (of which tribe the existing forests are chiefly composed), nor +do they resemble that class of exceptions to the common form of the leaves of +the Eucalyptus, which occur in this island. The travertin containing this +remnant of a lost vegetation, is of a pale yellow colour, hard, and in parts +even crystalline; but not compact, and is everywhere penetrated by minute, +tortuous, cylindrical pores. It contains a very few pebbles of quartz, and +occasionally layers of chalcedonic nodules, like those of chert in our +Greensand. From the pureness of this calcareous rock, it has been searched for +in other places, but has never been found. From this circumstance, and from the +character of the deposit, it was probably formed by a calcareous spring +entering a small pool or narrow creek. The strata have subsequently been tilted +and fissured; and the surface has been covered by a singular mass, with which, +also, a large fissure has been filled up, formed of balls of trap embedded in a +mixture of wacke and a white, earthy, alumino-calcareous substance. Hence it +would appear, as if a volcanic eruption had taken place on the borders of the +pool, in which the calcareous matter was depositing, and had broken it up and +drained it. +</p> + +<h3>ELEVATION OF THE LAND.</h3> + +<p> +Both the eastern and western shores of the bay, in the neighbourhood of Hobart +Town, are in most parts covered to the height of thirty feet above the level of +high-water mark, with broken shells, mingled with pebbles. The colonists +attribute these shells to the aborigines having carried them up for food: +undoubtedly, there are many large mounds, as was pointed out to me by Mr. +Frankland, which have been thus formed; but I think from the numbers of the +shells, from their frequent small size, from the manner in which they are +thinly scattered, and from some appearances in the form of the land, that we +must attribute the presence of the greater number to a small elevation of the +land. On the shore of Ralph Bay (opening into Storm Bay) I observed a +continuous beach about fifteen feet above high-water mark, clothed with +vegetation, and by digging into it, pebbles encrusted with Serpulae were found: +along the banks, also, of the river Derwent, I found a bed of broken sea-shells +above the surface of the river, and at a point where the water is now much too +fresh for sea-shells to live; but in both these cases, it is just possible, +that before certain spits of sand and banks of mud in Storm Bay were +accumulated, the tides might have risen to the height where we now find the +shells. ( It would appear that some changes are now in progress in Ralph Bay, +for I was assured by an intelligent farmer, that oysters were formerly abundant +in it, but that about the year 1834 they had, without any apparent cause, +disappeared. In the “Transactions of the Maryland Academy” volume 1 +part 1 page 28 there is an account by Mr. Ducatel of vast beds of oysters and +clams having been destroyed by the gradual filling up of the shallow lagoons +and channels, on the shores of the southern United States. At Chiloe, in South +America, I heard of a similar loss, sustained by the inhabitants, in the +disappearance from one part of the coast of an edible species of Ascidia.) +</p> + +<p> +Evidence more or less distinct of a change of level between the land and water, +has been detected on almost all the land on this side of the globe. Captain +Grey, and other travellers, have found in Southern Australia upraised shells, +belonging either to the recent, or to a late tertiary period. The French +naturalists in Baudin’s expedition, found shells similarly circumstanced +on the S.W. coast of Australia. The Rev. W.B. Clarke finds proofs of the +elevation of the land, to the amount of 400 feet, at the Cape of Good Hope. +(“Proceedings of the Geological Society” volume 3 page 420.) In the +neighbourhood of the Bay of Islands in New Zealand, I observed that the shores +were scattered to some height, as at Van Diemen’s Land, with sea-shells, +which the colonists attribute to the natives. (I will here give a catalogue of +the rocks which I met with near the Bay of Islands, in New Zealand:—1st, +Much basaltic lava, and scoriform rocks, forming distinct craters;—2nd, A +castellated hill of horizontal strata of flesh-coloured limestone, showing when +fractured distinct crystalline facets: the rain has acted on this rock in a +remarkable manner, corroding its surface into a miniature model of an Alpine +country: I observed here layers of chert and clay ironstone; and in the bed of +a stream, pebbles of clay-slate;—3rd, The shores of the Bay of Islands +are formed of a feldspathic rock, of a bluish-grey colour, often much +decomposed, with an angular fracture, and crossed by numerous ferruginous +seams, but without any distinct stratification or cleavage. Some varieties are +highly crystalline, and would at once be pronounced to be trap; others +strikingly resembled clay-slate, slightly altered by heat: I was unable to form +any decided opinion on this formation.) Whatever may have been the origin of +these shells, I cannot doubt, after having seen a section of the valley of the +Thames River (37 degrees S.), drawn by the Rev. W. Williams, that the land has +been there elevated: on the opposite sides of this great valley, three +step-like terraces, composed of an enormous accumulation of rounded pebbles, +exactly correspond with each other: the escarpment of each terrace is about +fifty feet in height. No one after having examined the terraces in the valleys +on the western shores of South America, which are strewed with sea-shells, and +have been formed during intervals of rest in the slow elevation of the land, +could doubt that the New Zealand terraces have been similarly formed. I may +add, that Dr. Dieffenbach, in his description of the Chatham Islands +(“Geographical Journal” volume 11 pages 202, 205.) (S.W. of New +Zealand), states that it is manifest “that the sea has left many places +bare which were once covered by its waters.” +</p> + +<h3>KING GEORGE’S SOUND.</h3> + +<p> +This settlement is situated at the south-western angle of the Australian +continent: the whole country is granitic, with the constituent minerals +sometimes obscurely arranged in straight or curved laminae. In these cases, the +rock would be called by Humboldt, gneiss-granite, and it is remarkable that the +form of the bare conical hills, appearing to be composed of great folding +layers, strikingly resembles, on a small scale, those composed of +gneiss-granite at Rio de Janeiro, and those described by Humboldt at Venezuela. +These plutonic rocks are, in many places, intersected by trappean-dikes; in one +place, I found ten parallel dikes ranging in an E. and W. line; and not far off +another set of eight dikes, composed of a different variety of trap, ranging at +right angles to the former ones. I have observed in several primary districts, +the occurrence of systems of dikes parallel and close to each other. +</p> + +<h3>SUPERFICIAL FERRUGINOUS BEDS.</h3> + +<p> +The lower parts of the country are everywhere covered by a bed, following the +inequalities of the surface, of a honeycombed sandstone, abounding with oxides +of iron. Beds of nearly similar composition are common, I believe, along the +whole western coast of Australia, and on many of the East Indian islands. At +the Cape of Good Hope, at the base of the mountains formed of granite and +capped with sandstone, the ground is everywhere coated either by a +fine-grained, rubbly, ochraceous mass, like that at King George’s Sound, +or by a coarser sandstone with fragments of quartz, and rendered hard and heavy +by an abundance of the hydrate of iron, which presents, when freshly broken, a +metallic lustre. Both these varieties have a very irregular texture, including +spaces either rounded or angular, full of loose sand: from this cause the +surface is always honeycombed. The oxide of iron is most abundant on the edges +of the cavities, where alone it affords a metallic fracture. In these +formations, as well as in many true sedimentary deposits, it is evident that +iron tends to become aggregated, either in the form of a shell, or of a +network. The origin of these superficial beds, though sufficiently obscure, +seems to be due to alluvial action on detritus abounding with iron. +</p> + +<h3>SUPERFICIAL CALCAREOUS DEPOSIT.</h3> + +<p> +A calcareous deposit on the summit of Bald Head, containing branched bodies, +supposed by some authors to have been corals, has been celebrated by the +descriptions of many distinguished voyagers. (I visited this hill, in company +with Captain Fitzroy, and we came to a similar conclusion regarding these +branching bodies.) It folds round and conceals irregular hummocks of granite, +at the height of 600 feet above the level of the sea. It varies much in +thickness; where stratified, the beds are often inclined at high angles, even +as much as at thirty degrees, and they dip in all directions. These beds are +sometimes crossed by oblique and even-sided laminae. The deposit consists +either of a fine, white calcareous powder, in which not a trace of structure +can be discovered, or of exceedingly minute, rounded grains, of brown, +yellowish, and purplish colours; both varieties being generally, but not +always, mixed with small particles of quartz, and being cemented into a more or +less perfect stone. The rounded calcareous grains, when heated in a slight +degree, instantly lose their colours; in this and in every other respect, +closely resembling those minute, equal- sized particles of shells and corals, +which at St. Helena have been drifted up the side of the mountains, and have +thus been winnowed of all coarser fragments. I cannot doubt that the coloured +calcareous particles here have had a similar origin. The impalpable powder has +probably been derived from the decay of the rounded particles; this certainly +is possible, for on the coast of Peru, I have traced LARGE UNBROKEN shells +gradually falling into a substance as fine as powdered chalk. Both of the +above-mentioned varieties of calcareous sandstone frequently alternate with, +and blend into, thin layers of a hard substalagmitic rock, which, even when the +stone on each side contains particles of quartz, is entirely free from them (I +adopt this term from Lieutenant Nelson’s excellent paper on the Bermuda +Islands “Geolog. Trans.” volume 5 page 106, for the hard, compact, +cream- or brown- coloured stone, without any crystalline structure, which so +often accompanies superficial calcareous accumulations. I have observed such +superficial beds, coated with substalagmitic rock, at the Cape of Good Hope, in +several parts of Chile, and over wide spaces in La Plata and Patagonia. Some of +these beds have been formed from decayed shells, but the origin of the greater +number is sufficiently obscure. The causes which determine water to dissolve +lime, and then soon to redeposit it, are not, I think, known. The surface of +the substalagmitic layers appears always to be corroded by the rain-water. As +all the above-mentioned countries have a long dry season, compared with the +rainy one, I should have thought that the presence of the substalagmitic was +connected with the climate, had not Lieutenant Nelson found this substance +forming under sea-water. Disintegrated shell seems to be extremely soluble; of +which I found good evidence, in a curious rock at Coquimbo in Chile, which +consisted of small, pellucid, empty husks, cemented together. A series of +specimens clearly showed that these husks had originally contained small +rounded particles of shells, which had been enveloped and cemented together by +calcareous matter (as often happens on sea-beaches), and which subsequently had +decayed, and been dissolved by water, that must have penetrated through the +calcareous husks, without corroding them,—of which processes every stage +could be seen.): hence we must suppose that these layers, as well as certain +vein- like masses, have been formed by rain dissolving the calcareous matter +and re-precipitating it, as has happened at St. Helena. Each layer probably +marks a fresh surface, when the, now firmly cemented, particles existed as +loose sand. These layers are sometimes brecciated and re-cemented, as if they +had been broken by the slipping of the sand when soft. I did not find a single +fragment of a sea-shell; but bleached shells of the Helix melo, an existing +land species, abound in all the strata; and I likewise found another Helix, and +the case of an Oniscus. +</p> + +<p> +The branches are absolutely undistinguishable in shape from the broken and +upright stumps of a thicket; their roots are often uncovered, and are seen to +diverge on all sides; here and there a branch lies prostrate. The branches +generally consist of the sandstone, rather firmer than the surrounding matter, +with the central parts filled, either with friable, calcareous matter, or with +a substalagmitic variety; this central part is also frequently penetrated by +linear crevices, sometimes, though rarely, containing a trace of woody matter. +These calcareous, branching bodies, appear to have been formed by fine +calcareous matter being washed into the casts or cavities, left by the decay of +branches and roots of thickets, buried under drifted sand. The whole surface of +the hill is now undergoing disintegration, and hence the casts, which are +compact and hard, are left projecting. In calcareous sand at the Cape of Good +Hope, I found the casts, described by Abel, quite similar to these at Bald +Head; but their centres are often filled with black carbonaceous matter not yet +removed. It is not surprising, that the woody matter should have been almost +entirely removed from the casts on Bald Head; for it is certain, that many +centuries must have elapsed since the thickets were buried; at present, owing +to the form and height of the narrow promontory, no sand is drifted up, and the +whole surface, as I have remarked, is wearing away. We must, therefore, look +back to a period when the land stood lower, of which the French naturalists +(See M. Peron “Voyage” tome 1 page 204.) found evidence in upraised +shells of recent species, for the drifting on Bald Head of the calcareous and +quartzose sand, and the consequent embedment of the vegetable remains. There +was only one appearance which at first made me doubt concerning the origin of +the cast,—namely, that the finer roots from different stems sometimes +became united together into upright plates or veins; but when the manner is +borne in mind in which fine roots often fill up cracks in hard earth, and that +these roots would decay and leave hollows, as well as the stems, there is no +real difficulty in this case. Besides the calcareous branches from the Cape of +Good Hope, I have seen casts, of exactly the same forms, from Madeira* and from +Bermuda; at this latter place, the surrounding calcareous rocks, judging from +the specimens collected by Lieutenant Nelson, are likewise similar, as is their +subaerial formation. Reflecting on the stratification of the deposit on Bald +Head,—on the irregularly alternating layers of substalagmitic +rock,—on the uniformly sized, and rounded particles, apparently of +sea-shells and corals,—on the abundance of land-shells throughout the +mass,—and finally, on the absolute resemblance of the calcareous casts, +to the stumps, roots, and branches of that kind of vegetation, which would grow +on sand-hillocks, I think there can be no reasonable doubt, notwithstanding the +different opinion of some authors, that a true view of their origin has been +here given. +</p> + +<p class="footnote"> +*(Dr. J. Macaulay has fully described (“Edinb. New Phil. Journ.” +volume 29 page 350) the casts from Madeira. He considers (differently from Mr. +Smith of Jordan Hill) these bodies to be corals, and the calcareous deposit to +be of subaqueous origin. His arguments chiefly rest (for his remarks on their +structure are vague) on the great quantity of the calcareous matter, and on the +casts containing animal matter, as shown by their evolving ammonia. Had Dr. +Macaulay seen the enormous masses of rolled particles of shells and corals on +the beach of Ascension, and especially on coral-reefs; and had he reflected on +the effects of long-continued, gentle winds, in drifting up the finer +particles, he would hardly have advanced the argument of quantity, which is +seldom trustworthy in geology. If the calcareous matter has originated from +disintegrated shells and corals, the presence of animal matter is what might +have been expected. Mr. Anderson analysed for Dr. Macaulay part of a cast, and +he found it composed of:— +</p> + +<pre xml:space="preserve"> +Carbonate of lime......73.15 +Silica.................11.90 +Phosphate of lime.......8.81 +Animal matter...........4.25 +Sulphate of lime......a trace + 98.11) +</pre> + +<p> +Calcareous deposits, like these of King George’s Sound, are of vast +extent on the Australian shores. Dr. Fitton remarks, that “recent +calcareous breccia (by which term all these deposits are included) was found +during Baudin’s voyage, over a space of no less than twenty-five degrees +of latitude and an equal extent of longitude, on the southern, western, and +north-western coasts.” (For ample details on this formation consult Dr. +Fitton “Appendix to Captain King’s Voyage.” Dr. Fitton is +inclined to attribute a concretionary origin to the branching bodies: I may +remark, that I have seen in beds of sand in La Plata cylindrical stems which no +doubt thus originated; but they differed much in appearance from these at Bald +Head, and the other places above specified.) It appears also from M. Peron, +with whose observations and opinions on the origin of the calcareous matter and +branching casts mine entirely accord, that the deposit is generally much more +continuous than near King George’s Sound. At Swan River, Archdeacon Scott +states that in one part it extends ten miles inland. (“Proceedings of the +Geolog. Soc.” volume 1 page 320.) Captain Wickham, moreover, informs me +that during his late survey of the western coast, the bottom of the sea, +wherever the vessel anchored, was ascertained, by crowbars being let down, to +consist of white calcareous matter. Hence it seems that along this coast, as at +Bermuda and at Keeling Atoll, submarine and subaerial deposits are +contemporaneously in process of formation, from the disintegration of marine +organic bodies. The extent of these deposits, considering their origin, is very +striking; and they can be compared in this respect only with the great +coral-reefs of the Indian and Pacific Oceans. In other parts of the world, for +instance in South America, there are SUPERFICIAL calcareous deposits of great +extent, in which not a trace of organic structure is discoverable; these +observations would lead to the inquiry, whether such deposits may not, also, +have been formed from disintegrated shells and corals. +</p> + +<h3>CAPE OF GOOD HOPE.</h3> + +<p> +After the accounts given by Barrow, Carmichael, Basil Hall, and W.B. Clarke of +the geology of this district, I shall confine myself to a few observations on +the junction of the three principal formations. The fundamental rock is granite +(In several places I observed in the granite, small dark-coloured balls, +composed of minute scales of black mica in a tough basis. In another place, I +found crystals of black schorl radiating from a common centre. Dr. Andrew Smith +found, in the interior parts of the country, some beautiful specimens of +granite, with silvery mica radiating or rather branching, like moss, from +central points. At the Geological Society, there are specimens of granite with +crystallised feldspar branching and radiating in like manner.), overlaid by +clay-slate: the latter is generally hard, and glossy from containing minute +scales of mica; it alternates with, and passes into, beds of slightly +crystalline, feldspathic, slaty rock. This clay-slate is remarkable from being +in some places (as on the Lion’s Rump) decomposed, even to the depth of +twenty feet, into a pale-coloured, sandstone-like rock, which has been +mistaken, I believe, by some observers, for a separate formation. I was guided +by Dr. Andrew Smith to a fine junction at Green Point between the granite and +clay-slate: the latter at the distance of a quarter of a mile from the spot, +where the granite appears on the beach (though, probably, the granite is much +nearer underground), becomes slightly more compact and crystalline. At a less +distance, some of the beds of clay-slate are of a homogeneous texture, and +obscurely striped with different zones of colour, whilst others are obscurely +spotted. Within a hundred yards of the first vein of granite, the clay-slate +consists of several varieties; some compact with a tinge of purple, others +glistening with numerous minute scales of mica and imperfectly crystallised +feldspar; some obscurely granular, others porphyritic with small, elongated +spots of a soft white mineral, which being easily corroded, gives to this +variety a vesicular appearance. Close to the granite, the clay-slate is changed +into a dark-coloured, laminated rock, having a granular fracture, which is due +to imperfect crystals of feldspar, coated by minute, brilliant scales of mica. +</p> + +<p> +The actual junction between the granitic and clay-slate districts extends over +a width of about two hundred yards, and consists of irregular masses and of +numerous dikes of granite, entangled and surrounded by the clay- slate: most of +the dikes range in a N.W. and S.E. line, parallel to the cleavage of the slate. +As we leave the junction, thin beds, and lastly, mere films of the altered +clay-slate are seen, quite isolated, as if floating, in the coarsely +crystallised granite; but although completely detached, they all retain traces +of the uniform N.W. and S.E. cleavage. This fact has been observed in other +similar cases, and has been advanced by some eminent geologists (See M. Keilhau +“Theory on Granite” translated in the “Edinburgh New +Philosophical Journal” volume 24 page 402.), as a great difficulty on the +ordinary theory, of granite having been injected whilst liquified; but if we +reflect on the probable state of the lower surface of a laminated mass, like +clay-slate, after having been violently arched by a body of molten granite, we +may conclude that it would be full of fissures parallel to the planes of +cleavage; and that these would be filled with granite, so that wherever the +fissures were close to each other, mere parting layers or wedges of the slate +would depend into the granite. Should, therefore, the whole body of rock +afterwards become worn down and denuded, the lower ends of these dependent +masses or wedges of slate would be left quite isolated in the granite; yet they +would retain their proper lines of cleavage, from having been united, whilst +the granite was fluid, with a continuous covering of clay-slate. +</p> + +<p> +Following, in company with Dr. A. Smith, the line of junction between the +granite and the slate, as it stretched inland, in a S.E. direction, we came to +a place, where the slate was converted into a fine-grained, perfectly +characterised gneiss, composed of yellow-brown granular feldspar, of abundant +black brilliant mica, and of few and thin laminae of quartz. From the abundance +of the mica in this gneiss, compared with the small quantity and excessively +minute scales, in which it exists in the glossy clay-slate, we must conclude, +that it has been here formed by the metamorphic action—a circumstance +doubted, under nearly similar circumstances, by some authors. The laminae of +the clay-slate are straight; and it was interesting to observe, that as they +assumed the character of gneiss, they became undulatory with some of the +smaller flexures angular, like the laminae of many true metamorphic schists. +</p> + +<h3>SANDSTONE FORMATION.</h3> + +<p> +This formation makes the most imposing feature in the geology of Southern +Africa. The strata are in many parts horizontal, and attain a thickness of +about two thousand feet. The sandstone varies in character; it contains little +earthy matter, but is often stained with iron; some of the beds are very +fine-grained and quite white; others are as compact and homogeneous as quartz +rock. In some places I observed a breccia of quartz, with the fragments almost +dissolved in a siliceous paste. Broad veins of quartz, often including large +and perfect crystals, are very numerous; and it is evident in nearly all the +strata, that silica has been deposited from solution in remarkable quantity. +Many of the varieties of quartzite appeared quite like metamorphic rocks; but +from the upper strata being as siliceous as the lower, and from the undisturbed +junctions with the granite, which in many places can be examined, I can hardly +believe that these sandstone-strata have been exposed to heat. (The Rev. W.B. +Clarke, however, states, to my surprise (“Geolog. Proceedings” +volume 3 page 422), that the sandstone in some parts is penetrated by granitic +dikes: such dikes must belong to an epoch altogether subsequent to that when +the molten granite acted on the clay-slate.) On the lines of junction between +these two great formations, I found in several places the granite decayed to +the depth of a few inches, and succeeded, either by a thin layer of ferruginous +shale, or by four or five inches in thickness of the re-cemented crystals of +the granite, on which the great pile of sandstone immediately rested. +</p> + +<p> +Mr. Schomburgk has described (“Geographical Journal” volume 10 page +246.) a great sandstone formation in Northern Brazil, resting on granite, and +resembling to a remarkable degree, in composition and in the external form of +the land, this formation of the Cape of Good Hope. The sandstones of the great +platforms of Eastern Australia, which also rest on granite, differ in +containing more earthy and less siliceous matter. No fossil remains have been +discovered in these three vast deposits. Finally, I may add that I did not see +any boulders of far-transported rocks at the Cape of Good Hope, or on the +eastern and western shores of Australia, or at Van Diemen’s Land. In the +northern island of New Zealand, I noticed some large blocks of greenstone, but +whether their parent rock was far distant, I had no opportunity of determining. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap08"></a>INDEX TO VOLCANIC ISLANDS.</h2> + +<p> +Abel, M., on calcareous casts at the Cape of Good Hope. +</p> + +<p> +Abingdon island. +</p> + +<p> +Abrolhos islands, incrustation on. +</p> + +<p> +Aeriform explosions at Ascension. +</p> + +<p> +Albatross, driven from St. Helena. +</p> + +<p> +Albemarle island. +</p> + +<p> +Albite, at the Galapagos archipelago. +</p> + +<p> +Amygdaloidal cells, half filled. +</p> + +<p> +Amygdaloids, calcareous origin of. +</p> + +<p> +Ascension, arborescent incrustation on rocks of. -absence of dikes, freedom +from volcanic action, and state of lava-streams. +</p> + +<p> +Ascidia, extinction of. +</p> + +<p> +Atlantic Ocean, new volcanic focus in. +</p> + +<p> +Augite, fused. +</p> + +<p> +Australia. +</p> + +<p> +Azores. +</p> + +<p> +Bahia in Brazil, dikes at. +</p> + +<p> +Bailly, M., on the mountains of Mauritius. +</p> + +<p> +Bald Head. +</p> + +<p> +Banks’ Cove. +</p> + +<p> +Barn, The, St. Helena. +</p> + +<p> +Basalt, specific gravity of. +</p> + +<p> +Basaltic coast-mountains at Mauritius. -at St. Helena. -at St. Jago. +</p> + +<p> +Beaumont, M. Elie de, on circular subsidences in lava. -on dikes indicating +elevation. -on inclination of lava-streams. -on laminated dikes. +</p> + +<p> +Bermuda, calcareous rocks of. +</p> + +<p> +Beudant, M., on bombs. -on jasper. -on laminated trachyte. -on obsidian of +Hungary. -on silex in trachyte. +</p> + +<p> +Bole. +</p> + +<p> +Bombs, volcanic. +</p> + +<p> +Bory St. Vincent, on bombs. +</p> + +<p> +Boulders, absence in Australia and Cape of Good Hope. +</p> + +<p> +Brattle island. +</p> + +<p> +Brewster, Sir D., on a calcareo-animal substance. -on decomposed glass. +</p> + +<p> +Brown, Mr. R., on extinct plants from Van Diemen’s land. -on sphaerulitic +bodies in silicified wood. +</p> + +<p> +Buch, Von, on cavernous lava. -on central volcanoes. -on crystals sinking in +obsidian. -on laminated lava. -on obsidian streams. -on olivine in basalt. -on +superficial calcareous beds in the Canary islands. +</p> + +<p> +Calcareous deposit at St. Jago affected by heat. -fibrous matter, entangled in +streaks in scoriae. -freestone at Ascension. -incrustations at Ascension. +-sandstone at St. Helena. -superficial beds at King George’s sound. +</p> + +<p> +Cape of Good Hope. +</p> + +<p> +Carbonic acid, expulsion of, by heat. +</p> + +<p> +Carmichael, Capt., on glassy coatings to dikes. +</p> + +<p> +Casts, calcareous, of branches. +</p> + +<p> +Chalcedonic nodules. +</p> + +<p> +Chalcedony in basalt and in silicified wood. +</p> + +<p> +Chatham island. +</p> + +<p> +Chlorophaeite. +</p> + +<p> +Clarke, Rev. W., on the Cape of Good Hope. +</p> + +<p> +Clay-slate, its decomposition and junction with granite at the Cape of Good +Hope. +</p> + +<p> +Cleavage of clay-slate in Australia. +</p> + +<p> +Cleavage, cross, in sandstone. +</p> + +<p> +Coast denudation at St. Helena. +</p> + +<p> +Columnar basalt. +</p> + +<p> +“Comptes Rendus,” account of volcanic phenomena in the Atlantic. +</p> + +<p> +Concepcion, earthquake of. +</p> + +<p> +Concretions in aqueous and igneous rocks compared. -in tuff. -of obsidian. +</p> + +<p> +Conglomerate, recent, at St. Jago. +</p> + +<p> +Coquimbo, curious rock of. +</p> + +<p> +Corals, fossil, from Van Diemen’s Land. +</p> + +<p> +Crater, segment of, at the Galapagos. -great central one at St. Helena. +-internal ledges round, and parapet on. +</p> + +<p> +Craters, basaltic, at Ascension. -form of, affected by the trade wind. -of +elevation. -of tuff at Terceira. -of tuff at the Galapagos archipelago. -their +breached state. -small basaltic at St. Jago. —at the Galapagos +archipelago. +</p> + +<p> +Crystallisation favoured by space. +</p> + +<p> +Dartigues, M., on sphaerulites. +</p> + +<p> +Daubeny, Dr., on a basin-formed island. -on fragments in trachyte. +</p> + +<p> +D’Aubuisson on hills of phonolite. -on the composition of obsidian. -on +the lamination of clay-slate. +</p> + +<p> +De la Beche, Sir H., on magnesia in erupted lime. -on specific gravity of +limestones. +</p> + +<p> +Denudation of coast at St. Helena. +</p> + +<p> +Diana’s Peak, St. Helena. +</p> + +<p> +Dieffenbach, Dr., on the Chatham Islands. +</p> + +<p> +Dikes, truncated, on central crateriform ridge of St. Helena. -at St. Helena; +number of; coated by a glossy layer; uniform thickness of. -great parallel ones +at St. Helena. -not observed at Ascension. -of tuff. -of trap in the plutonic +series. -remnants of, extending far into the sea round St. Helena. +</p> + +<p> +Dislocations at Ascension. -at St. Helena. +</p> + +<p> +Distribution of volcanic islands. +</p> + +<p> +Dolomieu, on decomposed trachyte. -on laminated lava. -on obsidian. +</p> + +<p> +Dree, M., on crystals sinking in lava. +</p> + +<p> +Dufrenoy, M., on the composition of the surface of certain lava-streams. -on +the inclination of tuff-strata. +</p> + +<p> +Eggs of birds embedded at St. Helena. -of turtle at Ascension. +</p> + +<p> +Ejected fragments at Ascension. -at the Galapagos archipelago. +</p> + +<p> +Elevation of St. Helena. -the Galapagos archipelago. -Van Diemen’s Land, +Cape of Good Hope, New Zealand, Australia, and Chatham island. -of volcanic +islands. +</p> + +<p> +Ellis, Rev. W., on ledges within the great crater at Hawaii. -on marine remains +at Otaheite. +</p> + +<p> +Eruption, fissures of. +</p> + +<p> +Extinction of land-shells at St. Helena. +</p> + +<p> +Faraday, Mr., on the expulsion of carbonic acid gas. +</p> + +<p> +Feldspar, fusibility of. -in radiating crystals. -Labrador, ejected. +</p> + +<p> +Feldspathic lavas. -at St. Helena. -rock, alternating with obsidian. +-lamination, and origin of. +</p> + +<p> +Fernando Noronha. +</p> + +<p> +Ferruginous superficial beds. +</p> + +<p> +Fibrous calcareous matter at St. Jago. +</p> + +<p> +Fissures of eruption. +</p> + +<p> +Fitton, Dr., on calcareous breccia. +</p> + +<p> +Flagstaff Hill, St. Helena. +</p> + +<p> +Fleurian de Bellevue on sphaerulites. +</p> + +<p> +Fluidity of lavas. +</p> + +<p> +Forbes, Professor, on the structure of glaciers. +</p> + +<p> +Fragments ejected at Ascension. -at the Galapagos archipelago. +</p> + +<p> +Freshwater Bay. +</p> + +<p> +Fuerteventura (Feurteventura), calcareous beds of. +</p> + +<p> +Galapagos archipelago. -parapets round craters. +</p> + +<p> +Gay Lussac, on the expulsion of carbonic acid gas. +</p> + +<p> +Glaciers, their structure. +</p> + +<p> +Glossiness of texture, origin of. +</p> + +<p> +Gneiss, derived from clay-slate. -with a great embedded fragment. +</p> + +<p> +Gneiss-granite, form of hills of. +</p> + +<p> +Good Hope, Cape of. +</p> + +<p> +Gorges, narrow, at St. Helena. +</p> + +<p> +Granite, junction with clay-slate, at the Cape of Good Hope. +</p> + +<p> +Granitic ejected fragments. +</p> + +<p> +Gravity, specific, of lavas. +</p> + +<p> +Gypsum, at Ascension. -in volcanic strata at St. Helena. -on surface of the +ground at ditto. +</p> + +<p> +Hall, Sir J., on the expulsion of carbonic acid gas. +</p> + +<p> +Heat, action of, on calcareous matter. +</p> + +<p> +Hennah, Mr., on ashes at Ascension. +</p> + +<p> +Henslow, Prof., on chalcedony. +</p> + +<p> +Hoffmann, on decomposed trachyte. +</p> + +<p> +Holland, Dr., on Iceland. +</p> + +<p> +Horner, Mr., on a calcareo-animal substance. -on fusibility of feldspar. +</p> + +<p> +Hubbard, Dr., on dikes. +</p> + +<p> +Humboldt on ejected fragments. -on obsidian formations. -on parapets round +craters. -on sphaerulites. +</p> + +<p> +Hutton on amygdaloids. +</p> + +<p> +Hyalite in decomposed trachyte. +</p> + +<p> +Iceland, stratification of the circumferential hills. +</p> + +<p> +Islands, volcanic, distribution of. -their elevation. +</p> + +<p> +Incrustation, on St. Paul’s rocks. +</p> + +<p> +Incrustations, calcareous, at Ascension. +</p> + +<p> +Jago, St. +</p> + +<p> +James island. +</p> + +<p> +Jasper, origin of. +</p> + +<p> +Jonnes, M. Moreau de, on craters affected by wind. +</p> + +<p> +Juan Fernandez. +</p> + +<p> +Keilhau, M., on granite. +</p> + +<p> +Kicker Rock. +</p> + +<p> +King George’s sound. +</p> + +<p> +Labrador feldspar, ejected. +</p> + +<p> +Lakes at bases of volcanoes. +</p> + +<p> +Lamination of volcanic rocks. +</p> + +<p> +Land-shells, extinct, at St. Helena. +</p> + +<p> +Lanzarote, calcareous beds of. +</p> + +<p> +Lava, adhesion to sides of a gorge. -feldspathic. -with cells +semi-amygdaloidal. +</p> + +<p> +Lavas, specific gravity of. +</p> + +<p> +Lava-streams blending together at St. Jago. -composition of surface of. +-differences in the state of their surfaces. -extreme thinness of. -heaved up +into hillocks at the Galapagos archipelago. -their fluidity. -with irregular +hummocks at Ascension. +</p> + +<p> +Lead, separation from silver. +</p> + +<p> +Lesson, M., on craters at Ascension. +</p> + +<p> +Leucite. +</p> + +<p> +Lime, sulphate of, at Ascension. +</p> + +<p> +Lonsdale, Mr., on fossil-corals from Van Diemen’s land. +</p> + +<p> +Lot, St. Helena. +</p> + +<p> +Lyell, Mr., on craters of elevation. -on embedded turtles’ eggs. -on +glossy coating to dikes. +</p> + +<p> +Macaulay, Dr., on calcareous casts at Madeira. +</p> + +<p> +MacCulloch, Dr., on an amygdaloid. -on chlorophaeite. -on laminated pitchstone. +</p> + +<p> +Mackenzie, Sir G., on cavernous lava-streams. -on glossy coatings to dikes. -on +obsidian streams. -on stratification in Iceland. +</p> + +<p> +Madeira, calcareous casts at. +</p> + +<p> +“Magazine, Nautical,” account of volcanic phenomena in the +Atlantic. +</p> + +<p> +Marekanite. +</p> + +<p> +Mauritius, crater of elevation of. +</p> + +<p> +Mica, in rounded nodules. -origin in metamorphic slate. -radiating form of. +</p> + +<p> +Miller, Prof., on ejected Labrador feldspar. -on quartz crystals in obsidian +beds. +</p> + +<p> +Mitchell, Sir T., on bombs. -on the Australian valleys. +</p> + +<p> +Mud streams at the Galapagos archipelago. +</p> + +<p> +Narborough island. +</p> + +<p> +Nelson, Lieut., on the Bermuda islands. +</p> + +<p> +New Caledonia. +</p> + +<p> +New Red sandstone, cross cleavage of. +</p> + +<p> +New South Wales. +</p> + +<p> +New Zealand. +</p> + +<p> +Nulliporae (fossil), resembling concretions. +</p> + +<p> +Obsidian, absent at the Galapagos archipelago. -bombs of. -composition and +origin of. -crystals of feldspar sink in. -its irruption from lofty craters. +-passage of beds into. -specific gravity of. -streams of. +</p> + +<p> +Olivine decomposed at St. Jago. -at Van Diemen’s land. -in the lavas at +the Galapagos archipelago. +</p> + +<p> +Oolitic structure of recent calcareous beds at St. Helena. +</p> + +<p> +Otaheite. +</p> + +<p> +Oysters, extinction of. +</p> + +<p> +Panza islands, laminated trachyte of. +</p> + +<p> +Pattinson, Mr., on the separation of lead and silver. +</p> + +<p> +Paul’s, St., rocks of. +</p> + +<p> +Pearlstone. +</p> + +<p> +Peperino. +</p> + +<p> +Peron, M., on calcareous rocks of Australia. +</p> + +<p> +Phonolite, hills of. -laminated. -with more fusible hornblende. +</p> + +<p> +Pitchstone. -dikes of. +</p> + +<p> +Plants, extinct. +</p> + +<p> +Plutonic rocks, separation of constituent parts of, by gravity. +</p> + +<p> +Porto Praya. +</p> + +<p> +Prevost, M. C., on rarity of great dislocations in volcanic islands. +</p> + +<p> +Prosperous hill, St. Helena. +</p> + +<p> +Pumice, absent at the Galapagos archipelago. -laminated. +</p> + +<p> +Puy de Dome, trachyte of. +</p> + +<p> +Quail island, St. Jago. +</p> + +<p> +Quartz, crystals of, in beds alternating with obsidian. -crystallised in +sandstone. -fusibility of. -rock, mottled from metamorphic action with earthy +matter. +</p> + +<p> +Red hill. +</p> + +<p> +Resin-like altered scoriae. +</p> + +<p> +Rio de Janeiro, gneiss of. +</p> + +<p> +Robert, M., on strata of Iceland. +</p> + +<p> +Rogers, Professor, on curved lines of elevation. +</p> + +<p> +Salses, compared with tuff craters. +</p> + +<p> +Salt deposited by the sea. -in volcanic strata. -lakes of, in craters. +</p> + +<p> +Sandstone of Brazil. -of the Cape of Good Hope. -platforms of, in New South +Wales. +</p> + +<p> +Schorl, radiating. +</p> + +<p> +Scrope, Mr. P., on laminated trachyte. -on obsidian. -on separation of trachyte +and basalt. -on silex in trachyte. -on sphaerulites. +</p> + +<p> +Seale, Mr., geognosy of St. Helena. -on dikes. -on embedded birds’ bones. +</p> + +<p> +Seale, on extinct shells of St. Helena. +</p> + +<p> +Sedgwick, Professor, on concretions. +</p> + +<p> +Septaria, in concretions in tuff. +</p> + +<p> +Serpulae on upraised rocks. +</p> + +<p> +Seychelles. +</p> + +<p> +Shells, colour of, affected by light. -from Van Diemen’s land. -land, +extinct, at St. Helena. -particles of, drifted by the wind at St. Helena. +</p> + +<p> +Shelly matter deposited by the waves. +</p> + +<p> +Siau, M., on ripples. +</p> + +<p> +Signal Post Hill. +</p> + +<p> +Silica, deposited by steam. -large proportion of, in obsidian. -specific +gravity of. +</p> + +<p> +Siliceous sinter. +</p> + +<p> +Smith, Dr. A., on junction of granite and clay-slate. +</p> + +<p> +Spallanzani on decomposed trachyte. +</p> + +<p> +Specific gravity of recent calcareous rocks and of limestone. -of lavas. +</p> + +<p> +Sphaerulites in glass and in silicified wood. -in obsidian. +</p> + +<p> +Sowerby, Mr. G.B., on fossil-shells from Van Diemen’s land. -from St. +Jago. -land-shells from St. Helena. +</p> + +<p> +St. Helena. -crater of elevation of. +</p> + +<p> +St. Jago, crater of elevation of. -effects of calcareous matter on lava. +</p> + +<p> +St. Paul’s rocks. +</p> + +<p> +Stokes, Mr., collections of sphaerulites and of obsidians. +</p> + +<p> +Stony-top, Little. -Great. +</p> + +<p> +Stratification of sandstone in New South Wales. +</p> + +<p> +Streams of obsidian. +</p> + +<p> +Stutchbury, Mr., on marine remains at Otaheite. +</p> + +<p> +Subsided space at Ascension. +</p> + +<p> +Tahiti. +</p> + +<p> +Talus, stratified, within tuff craters. +</p> + +<p> +Terceira. +</p> + +<p> +Tertiary deposit of St. Jago. +</p> + +<p> +Trachyte, absent at the Galapagos archipelago. -at Ascension. -at Terceira. +-decomposition of, by steam. -its lamination. -its separation from basalt. +-softened at Ascension. -specific gravity of. -with singular veins. +</p> + +<p> +Trap-dikes in the plutonic series. -at King George’s sound. +</p> + +<p> +Travertin at Van Diemen’s land. +</p> + +<p> +Tropic-bird, now rare, at St. Helena. +</p> + +<p> +Tuff, craters of. -their breached state. -peculiar kind of. +</p> + +<p> +Turner, Mr., on the separation of molten metals. +</p> + +<p> +Tyerman and Bennett on marine remains at Huaheine. +</p> + +<p> +Valleys, gorge-like, at St. Helena. -in New South Wales. -in St. Jago. +</p> + +<p> +Van Diemen’s land. +</p> + +<p> +Veins in trachyte. -of jasper. +</p> + +<p> +Vincent, Bory St., on bombs. +</p> + +<p> +Volcanic bombs. -island in process of formation in the Atlantic. -islands, +their distribution. +</p> + +<p> +Wacke, its passage into lava. +</p> + +<p> +Wackes, argillaceous. +</p> + +<p> +Webster, Dr., on a basin-formed island. -on gypsum at Ascension. +</p> + +<p> +White, Martin, on soundings. +</p> + +<p> +Wind, effects of, on the form of craters. +</p> + +</div><!--end chapter--> + +<div style='display:block;margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK VOLCANIC ISLANDS ***</div> +<div style='display:block;margin:1em 0;'>This file should be named 3054-h.htm or 3054-h.zip</div> +<div style='display:block;margin:1em 0;'>This and all associated files of various formats will be found in https://www.gutenberg.org/3/0/5/3054/</div> +<div style='display:block; margin:1em 0'> +Updated editions will replace the previous one—the old editions will +be renamed. +</div> + +<div style='display:block; margin:1em 0'> +Creating the works from print editions not protected by U.S. copyright +law means that no one owns a United States copyright in these works, +so the Foundation (and you!) can copy and distribute it in the United +States without permission and without paying copyright +royalties. 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